EDIT 5/4/2016: As I mentioned my workbench is using hardwood dinner table for the top, and well also a double layer on the front. So the front only sticks out 2" past the legs on my workbench. Well the design I've provided sticks out 3" because it is using two 2x4's stacked. This makes the bench more likely to tip forward. I suggest moving one of the 2x4's on the front to the back -- this will make it stick out 1 1/2" on both the front and back -- so it will be balanced and less likely to tip. I just made one 36" wide with the same dimensions I've provided in the sketchup file and I'm not happy how far it sticks out front -- I'll be moving one of the front 2 x 4's to the back.. rather making the back the new front. It will still have the same x and y dimensions on the top of the workbench so that's good -- just screw it back down after moving 2x4 to the back.
This is the workbench I built for my garage. It is ROCK SOLID for life. It doesn't wiggle in the slightest because of the design. You can get up on it and jump up and down, and it just won't wobble at all.
I got fed up looking at all the store bought "workbenches"; what a joke: they wobble like mad and they want between $125-400 for the junk! Worthless.. you need a very stable bench to do any serious work. I guess these manufacturers think most of us don't do anything important or significant -- how insulting!
You can make this workbench for around $50 since it's made of affordable 2x4's.
Cap it with whatever you like -- 3/4" MDF top would work well. (I cut up an old hardwood dinner table that I got for $25 and used it for the top -- had to countersink and screw down in the middle as well because it was a bit warped.. then I had to jack plane it .. MDF is easier.) Default dimensions on the top are 2x4 feet so you can just buy a 1/4 sheet 3/4" MDF from the hardware store for under $15, then slap it on and go. Plywood top is another way to go -- smoother and more level is better though that's why I suggested MDF.
Just open this file up in Sketchup (free program) and use the program's tape measure tool to get the dimensions for each cut -- that's what I did after I designed it and was starting to build it.
Start by building the legs (see photo #2). Then the next step would be to tie the to pairs of legs together with the 2x4's in the top middle. Then the two 2x4's in the bottom back. Then go from there, adding supports and more 2x4's.
I made the front of the workbench thicker to accommodate more clamping space and a stronger front for future installation of a woodworking vise.
If you need a good strong workbench, for serious work, I guarantee you'll be happy with this one.
What really makes this workbench is the leg construction. By screwing together two 2x4's it's cheaper than a 4x4 and you don't have to cut out the joints. Make sure when you construct the legs to put pressure on the wood forming the "full lap joints", while screwing down -- you want the joints to be as tight as possible. (See photo #2.)
I clamp and screw everything together with torx (6 pointed star) deck screws -- drilling pilots first with smaller bit. Don't bother with phillips head deck screws -- they will strip like mad (what a nightmare experience).
If there is any wobble/rocking after you build this, know it's just because the 4 feet aren't perfectly level. Make sure it's on level concrete/floor then figure out which feet are too long and sand them down a bit until perfectly level. Then enjoy your rock solid workbench.
In addition to the strength and affordability of this workbench, I really like the simplicity of constructing it: it's all 2x4 chop cuts -- very fast and easy with a compound miter saw. Chop & screw! That's it! :)
EDIT 04/10/2016: Another Thingiverse member re-made this workbench as a customizable OpenSCAD file. Details here: http://www.thingiverse.com/thing:1481334