Although push up bars are pretty affordable, I'm the sort of guy who'll make something rather than buy it provided I've got all of the materials lying around.

Step 1: Bill of Materials

What you'll need:
14" of 2x4, cut into (4) x 3.5" sections
- cut a "v" into each, .75" deep an 1.5" wide to keep the pipe from moving
14" of 2.25" galvanized steel pipe for hurricane fences
- cut into (2) x 7" sections (I used an angle grinder with abrasive cut off wheel, then ground the edges smooth)
28" of pipe hanging strap - .75" wide or thereabout, cut into 7"strips
Screws of some variety
- Drywall screws are usually my first choice, but I didn't want to drill pilot holes so I used some 1" self drilling metal screws that worked just fine.

Note: The metal pipe and pipe strap that I used are galvi - certainly not necessary. but since these'll be subjected to sweat it's good to mitigate rust.

Step 2: Assembly

It's pretty straight forward. I first attached the ends of the pipe strap on the outsides of the 2x4, then I put a screw closer to the pipe to get good tension and really secure the pipe to the 2x4.

Repeat this process till all ends of the pipe are cradled by the pieces of wood. Then you're done with the push up bars and ready to commence push ups.

Hurrah - I made these at TechShop in San Francisco. I'd feel a bit silly doing push ups there, but I don't feel silly making all kinds of funky stuff! See what you can make: http://www.techshop.ws/

Note: These push up bars are pretty stable when used on unfinished concrete. If I wanted to use them on some surface that has less grip (is a slicker surface), it may not be a bad idea to put some rubber feet on the bottom to prevent the push up bars from sliding around.