I haven't built my dream workbench yet, so I needed a large work table that I could set up quickly wherever I needed it. I have a pair of sawhorses that I use all the time, and an approximately 30"x30" plywood shelf that I put across them for a work table, but I want something bigger.
However, I didn't want to permanently alter my sawhorses too much because I use them for all sorts of things.
What I came up with is simple, quite sturdy, and can be taken down and set up very quickly.

Materials
2 sawhorses that I already had
4 simple 2x4 joist hangars (upc 0081942100607, I got mine at Menards).
8 nails
small diameter wood dowel (i used some 1/4" that I had)
pair of 2x4s of matching length (I used a pair of 4 foot and a pair of 6 foot ones for 2 sizes of work table)
sheet plywood of a size to go over your new table frame (I used a 3'x7' piece of 3/8" plywood that my neighbor was throwing out)

Tools
Hammer (to drive the nails)
Hacksaw (to cut off the nails and notch the dowels)
Power drill and bits (for the dowel holes in joist hangars and cross members)
Pliers (to bend the cut nails)
Metal file (to take the rough edges off the cut nails and the holes drilled in the joist hangars)

Step 1: Prepare the Joist Hangars.

First, I did some minor cosmetic work to the joist hangars. The bent metal part shown in the photo is not needed, so I hammered it flat so that it is out of the way.

Next, I found some nails in my collection which fit fairly closely in the holes indicated. These will go into the side of the sawhorse to hold the joist hangars.

Decide the most practical place for the cross members. The plywood sheet I had for a table top was only about 30" wide so I needed to put my cross members inside the legs. It is very stable this way, but if you were going to use a full 48" sheet, you might want to put them outside the legs. Of course every set of sawhorses will be different.

The most important thing is to align the nails for the joist hangar so that the top edge of the vertical cross members will be flush with the horizontal top edge of the sawhorses. The best way to do this is to hold a small piece of scrap 2x4 in the hangar while you hold the hangar against the sawhorse. Align the hangar and mark the side of the sawhorse with a pencil through the top two holes in the hangar. You should have already measured so that the hangars will be evenly spaced.

Hammer a nail in at each pencil mark, leaving about 1/2" out of the board. Try to drive them in as straight as possible so that you don't split the sawhorse top or come out the bottom. Then cut the heads off the nails with the hacksaw and file off the sharp edges. With the pliers, bend each of the nails up about 20 degrees; not so much to bind or raise the hangars above flush, but enough to hold the hangar from slipping off the nail. The weight of the cross members and the top will hold everything just fine.

The last thing you need to do is to mark the center of the bottom surface of the joist hangar for a hole for the dowel pins. I made mine just bigger than the diameter of the dowel so that you can find the hole fairly easily while you are setting up. Drill the hole in all four joist hangars and file down the rough edges.

Step 2: Finish the Cross Members.

For the cross members, I tried to pick out straight and true pairs of 2x4s. I actually got 2 matching 6' ones and one 8' one that I cut exactly in two for a roughly 4' pair.

I set up the two sawhorses and hung the hangars on them. After placing the 2x4s in the hangars, I marked the location for the dowels with a pencil from the bottom through the holes. I marked both pair of cross members and then drilled the 15/64" holes with the drill approximately 1/2" to 3/4" deep.

To make the dowel pins, take the 1/4" dowel and holding it (preferably in a vise), use the hacksaw to cut a thin notch as close to centered about 1/2" down from the end, as shown in the photo. Note: it is much easier to cut the notch on the long dowel and then cut it to length than to cut off a 1.5" piece of dowel and then notch it. You are basically making a Tinker Toy piece (kudos if you remember those) that will fit snugly in the cross member hole, but can be removed for storage or to switch between different sets of cross members.

Cut off the dowel about 1.5" long. Do this 3 more times so that you have a dowel pin for each end of each cross member.

Insert the dowel pins and assemble the sawhorses and cross members, lining up the dowel pins through the holes as shown.

Step 3: That's It.

That's it.

Place the piece of plywood sheet on top and you have a quick temporary work table that is quite stable and can support quite a bit of weight.

You could also use it without the top when cutting plywood sheet or long boards to keep them from sagging while you cut. Just make sure you line things up so that your cut won't go across the joist hangars or the sawhorse hardware (or deep enough to hit the nails).

Obviously, it is not as sturdy, or strong as a real workbench. I wouldn't do a lot of pounding or fine assembly on this, but it works great for spreading out the parts of a big tear down or assembly project.