I got an XYZ DaVinci 1.0a, hacked to have Repetier 0.92mod firmware, and wanted to use it as a plotter / cutter as well as a 3D printer. Some use cases:

plotting with fabric markers on T-shirts; the main point is to make coloring-sheet T-shirts our 4-year-old can color with fabric markers
drawing traces on circuit boards for etching
cutting gum paste for cake decorating
cutting paper or vinyl with a drag-knife.

It was important to me that the printer mod be easily and quickly installable and uninstallable, so one can quickly switch between plotting, cutting and 3D printing.

The software side of this is a Python script / Inkscape plugin that I wrote to convert SVG / HPGL path files into gcode that the printer can print.

Materials:

M3 screws and nuts
ABS filament
pen or marker (for plotter)
pins, plywood, screws and bungee cord (for extended print platform)
cutter holder and blades (I used this one) (for cutter)
Cricut adhesive mat (for cutter)

My total cost was under $20, not counting the printer.

Step 1: Attachment Clip and Springy Tool Holder

For versatility and easy of printing, I went with a two-part approach:

attachment clip for the print head
springy tool holder for pen or cutter.

The attachment clip provides a mounting surface, with screw holes or gluable mounting holes, that clips onto the print head. The tool holder then screws or glues to it.

The attachment clip needs to be customized for your particular print head. Mine is for the XYZ DaVinci 1.0a. You may be able to use my OpenSCAD file to to customize for other printers. Experience shows that you'll have to measure, print, modify, print again.

I've used three tool holders. All of them need to be printed in ABS, and get their springiness from built-in ABS springs. The springiness is essential to compensate for imperfections in the surface (though you also may get some bonus springiness from your bed's leveling springs--but that may screw up your leveling).

Two-piece very springy pen holder that I remixed from one that EGoe made for the ShapeOko.
Low-profile pen holder that I designed from scratch.
Roland-style blade mount holder that I also designed from scratch. The bird-shaped spring here is just designed to press on the blade mount's upper pin. You may need to change the dimensions slightly to fit your blade mount.

As long as the mounting screw/glue holes on your attachment clip has the same placement as on mine, you should be able to just print my tool holders without change. I used ABS juice (ABS dissolved in acetone) to attach the two piece holder to the attachment clip. The bond is surprisingly strong. The rest uses M3 screws and nuts.

The dimensions my pen holders are designed to hold up to a 15mm fabric marker, but you can use Thingiverse's Customizer to change the size. All of them are designed to use M3 screws and nuts to hold the tool in place. There are screw nut holes that are printed by the file. On my printer, these tend to come out very snug and I had to run a drill bit (sometimes just handheld) through the screw holes, and then use a screw to force a nut into the hole. The nice thing about that is that the nuts then tend to stay in the hole.

The two-piece pen holder is really cool looking, but takes up a lot of space, which limits your print area. Hence I designed my low-profile holder instead.

Step 2: Extended Print Platform

On my printer, because the tool is off to one side of the print bed, a large chunk of the print bed can't be used for drawing and cutting. However, there is also a chunk of space that could be used if the print bed were large. In my scrap collection, I had the remains of a plywood jigsaw puzzle, and I glued on some extra pieces of wood that made it into an extended print platform, which ties on with bungee cord. A bonus of this is that I can use thumbtacks to attach work to the wood. This is ugly and hackish, but worked.

I wouldn't use this for cutting, as it's going to be hard to make the plywood as level as a glass bed would be, and leveling is much more crucial for cutting.

Step 3: Calibrating

Level your printer bed. If you're going to be cutting, level it very carefully.

Attach your clip and tool holder, and insert the tool you will use. The calibration will be different for different combinations, and the Z-calibration will need to be done each time you level, as the heights will change slightly.

Make sure you can home your printer without the clip, holder or tool bumping into the bed. If not, you will need to do some careful hardware/software working around.

Now, use your printer panel or software (I used Repetier Host) in manual mode to move the printhead around.

First, move the printhead away from the bed for safety. Then locate the minimum and maximum XY coordinates of the available print area (make sure printer can move freely over it without bumping into any clips that you will use to hold the work in place). On my printer with the two-piece holder, the coordinates are minX=10, minY=8, maxX=192 and maxY=150, all in millimeters.

Now comes the crucial point. This is so crucial, I recommend doing it before every print. Find the Z coordinate that the tool will be in while drawing and cutting. To do this, attach whatever you want to draw on (paper, fabric, etc.) on the bed, or attach the cutting mat. Then very slowly lower the tool until it is at the desired height. To prevent marking the object when drawing, you can put a piece of aluminum foil between the pen/marker and the object.

For drawing, you want the pen to exert some pressure on the drawing surface. You'll need to experiment to get the right amount of pressure. For cutting, you want the very tip of the blade to slightly penetrate the cutting mat.

Make a note of the Z coordinate.

Then check how many millimeters you need to lift the tool for movement. Typically two or three millimeters of lift will do. The less you lift, the faster the print will be.

Finally, if you're cutting, measure the "tool offset". This is the distance from the swiveling knife pivot to the far edge of the knife. Typically, this will just be the width of the knife.

You will now have this data, all in millimeters:

minimum and maximum X and Y coordinates
work Z coordinate
distance to lift tool
tool offset (if cutting)

Step 4: Preparing Gcode With Inkscape

There is a pretty good chance you can use the bundled gcodetools extension to drive your printer as a plotter, but I ended up writing my own python script and extension.

1. Make sure you have Inkscape installed.

2. Download the latest release of my gcodeplot scripts. Drag the contents of the zip file into your Inkscape's extensions directory. E.g., C:\Program Files\Inkscape\share\extensions in a typical Windows installation (you will get a UAC prompt in Windows, and will need to use an account with admin privileges; if you don't have such an account, do a single-user install of Inkscape).

3. Restart or start Inkscape.

4. Load the SVG file you want to draw or cut into Inskcape. The lower-left corner of the page in Inkscape corresponds to the minimum X and Y coordinates you measured in the previous step. Make sure your images fit into the print area.

5. Convert the drawing to a path: ctrl-A to select everything, then Path | Object to Path.

6. Save to gcode:

File | Save As...
Select: "3-axis gcode plotter" in the "Save as type" dropdown (if it doesn't show up, gcodeplot hasn't been installed properly)
After pressing "Save", you will have a complicated window with many settings.
General tab:
- Tool mode: choose "drawing" or "cutting" to start off
- Left x-coordinate, lower y-coordinate: the minimum X and Y coordinates from previous step
- Right x-coordinate, upper y-coordinate: the maximum X and Y coordinates from previous step
- Work z-coordinate, from previous step
- Lift z-height: distance to lift tool relative to work for movement, from previous step
- Parking z-height: a good safe lift distance relative to work, to miss all clips, etc. (20mm default should be fine)
- Serial port: leave blank
Fitting and Extracting tab:
- I recommend the defaults
Drawing Settings tab (if drawing):
- Shading threshold: 0 if you don't want to shade colored areas; 1 if you want to shade all colored areas
Cutting Settings tab (if cutting):
- Tool Offset: set as measured in previous step
- Overcut: probably set to the same value as the Tool Offset
Press OK.

Step 5: Drawing or Cutting

Make sure your work is attached well enough to the print bed. Here are some hints:

if you use any clips, make sure that the print head or pen won't crash into them
make sure bed is not being heated
for drawing on fabric with a fabric marker, make sure the fabric is very strongly attached along the edges and well stretched; otherwise, the marker will pull the fabric
for drawing on paper, you can just clip the paper to the bed or tape it with painter's tape; with a ball-point pen, I've had good luck just drawing on a sticky note that was stuck to the bed (it was a bit off on the side not glued down, but it still worked)
I'd attach CDs or PCBs (I haven't tried PCBs) with painter's tape
for cutting, use a Cricut adhesive mat, cut to the correct size, and clipped to your bed; you can customize the clips that I used

Load the gcode file into your printer control program and print. I used Repetier Host. Kill the print (know what your printer's reset button sequence is) if anything goes wrong to avoid damage.

Step 6: More Software Stuff

While saving the gcode with my gcodeplot Inkscape extension, you can fill out a serial port name (e.g., "COM4") and have it simultaneously send the code to your printer. I don't recommend this when starting out as you have no control during sending, and the reliability is likely lower than using something like Repetier Host. But eventually you can try it.

If you have python installed, gcodeplot.py can also be used directly from the commandline, and applied to a path-only SVG file (i.e., an SVG file where objects were converted to paths with Select-All and then Path | Object-to-Path in Inkscape) or an HPGL file. Run python gcodeplot.py (assuming you have python in your path, and you're in the root gcodeplot directory) to list the options. One particularly useful option is adjusting the x/y/z coordinates right inside gcodeplot.py. To do that, include --pause-at-start --send=portname in the commandline, e.g.:

python gcodeplot.py --pause-at-start --send=COM4 mydrawing.svg

The printer will start up, but before any printing starts, you will get a gcodeplot commandline that lets you move the printhead around. Here are some pause-mode commands:

xn: here n is a number (e.g., x7, x0 or x-4.3) in millimeters; moves printhead to absolute x coordinate n. You can also replace n by left, center or right, as well as by more complicated formulas like (left+center)/2. In all commands, if you use formulas, there must be no spaces in them. Also, there must be no space between x and the number.
yn and zn: same but for y and z axes; the special coordinates bottom and top (for the y axis), and work and safe (for the z axis) will be recognized
x+n: increments the x position by n. E.g., x+0.7 moves the printhead by 0.7 mm in the positive direction, and x+-0.7 by 0.7 in the negative direction. (Don't confuse with x-0.7 which moves to absolute coordinate -0.7)
y+n and z+n: same but for y and z axes
left=x: set the left value (minimum x coordinate) for the drawing to the current x coordinate (you can also do more complicated formulas, like left=x-3 or left=left-0.1; again, no spaces)
bottom=y: set the bottom value (minimum x coordinate) for the drawing to the current y coordinate
work=z: set the work z-value
Gxxx: send any G-command to the printer
c: continue printing

After you set the left, bottom and work values, the drawing/cutting will be adjusted to use them.

Finally, you can do multicolor plotting by using a pen control file. Make a text file, e.g., named pens.txt, with contents like this:

1 (0,0) red Red marker
2 (0,0) #00FF00 Green marker
3 (0,0) black Black marker
4 (0,2) yellow Highlighter

The general format is penNumber (offsetX,offsetY) svgColor Description:

penNumber: an integer--drawing will be sorted in order of penNumber
(offsetX,offsetY): used in case the pen is a different thickness from the pen you used when calibrating; for instance, if your pen tip sticks out 2mm further in the y-direction than the one you used when calibrating, put (0,2). If all the pens are the same size, use (0,0) for them all. (There is no out-of-bounds checking when you use offsets. Be careful.)
svgColor: pen color, specified in SVG color format; there are many named colors; you can also use #RRGGBB hex color names, as well as rgb(r,g,b) color names where r,g,b are integers fromI y 0 to 255 or percentages (e.g., rgb(100%,100%,100%)).

If you then include --pens=pens.txt on your gcodeplot.py commandline, gcodeplot.py will match the colors in the drawing to the closest colors in your pens file, and insert a pause command in the gcode for pen changes. The challenge is either making sure that the pens all stick out the same distance (you'll have to do this if you are using a 3rd party program like Repetier Host to send the gcode to the printer) or else re-callibrating the z-coordinate using gcodeplot's pause-mode commands (i.e., move the printhead to the correct work position with zn or z+n commands--with aluminum foil to protect the work--and then do work=z).

I have yet to test the multiple color feature, as I haven't had a use for it.