After seeing a lot of articles online about how terrible sitting all day is for your body, I decided to try a standing desk. And since most of the articles I read also said that standing all day was no fun, I didn't want to make the transition permanent. I looked for adjustable monitor mounts from Ergotron and Kangaroo, but ones with dual monitors and keyboard trays were $400-600. Plus, reviewers commented that the keyboard trays had a tendency to bounce since they're cantilevered. I knew I could do better for cheaper.

Monitor Parts list:
Monitor mount
2"x8"x6' board
18"-24" full extension, self-closing drawer slides, depending on your height (2)
1.5" angle brackets (4)
1" black iron pipe
flange
close nipple
elbow
3.5" nipple
end cap
straps (2)
1.5" long 3/8" dia. lag bolts (4)
1 1/4" drywall screws
a gate latch, I found this one at Lowe's. It's nice because the two sections are the same size.

Keyboard Parts List:
pine stair tread
1/5" square table legs (4)

magnetic drawer catches, non-locking (4)
1.5" wide hinges (4)
-or-
bronze folding table leg brackets (4)

Estimated cost: $150

NOTE: Stain and varnish all wood before construction. I did it the other way around and it took much longer than necessary, but I was too eager to put the parts together first.

Step 1: Measurements

First I needed to figure out what heights was comfortable to work at. This was my inelegant first attempt. For my 28" high desk and 5'10" body, having the keyboard 12" off the surface and the center of the monitor 33" off the surface was a sweet spot for me.

Step 2: Mount Construction

Cut a 6" piece of the long pine board for the base.
Cut 2 more pieces of the board to be the same length as whichever drawer slides you bought, plus 1/4". One of my pieces is much longer, but it's not necessary and makes the slides harder to mount.
Attach one of the pieces to the metal mount using the pipe straps and drywall screws. Attach it to the base board with the four angle brackets. I would recommend putting the two front brackets in the center of the board so you can mount the slides flush with the base, unlike what I've done.
Screw the drawer slides to the board. Use a spacer to make sure they're parallel and vertical.
Then attach the second board to the other side of the slides and you're almost done.

I used the six included screws and six extra drywall screws on each slide for extra strength.

Step 3: Monitor Mount Cont.

Next, mount the pipe flange to the board with the lag bolts. The center of the monitors will be about 3" above where the center of the flange is. You can see the holes where I made that mistake.
Assemble the pipes as in the picture, but leave the cap off for now.
With the mount in a horizontal position for ease of use, measure to where you want the "standing" position to be and attach the gate latch.
In your office, clamp the mount using the included thumb screws. You'll need about 2.5" of overhang to get a good grip.
Attach the mount and put the cap on the pipe.
Screw on your monitors.

Step 4: Keyboard Construction

Cut the stair tread to length. Mine is 30"
Attach either the magnets and hinges or the bronze brackets to the legs first and and then to the bottom of the board.