Equipment for Janda Sit-ups and other fun things.
Everything from bars to a belt squatting setup.
Building a Lifting Platform (Ironmind)
Randall J. Strossen
Now all you need is a set of bumper plates.
Building Your Own Set of Atlas Stones
Jason F. Keen
Making stones using an old - but effective - recipe of plaster, cement and water. And a couple of inflatable balls. A similar article appears at Body Results.
Some good discussion on the ins and outs of equipment, both store-bought and home-made.
Construction of a bouldering/traverse wall.
Plastic shopping bags, pipe, a tennis ball and of course lots and lots of duct tape.
A forum for DIY gymrats everywhere.
Building a spring loaded mat. Superb.
If you've ever considered the idea of having your own Glute-Ham Raise, take a look at this. While you're there, check out the home-made slammable medicine ball (based on instructions [.pdf, 1.21mb] from Pierre Augé).
Free instructions for building squat stands and plyometric boxes.
Some great articles here, including Make Your Own 200m Track [.pdf, 204kb]
Plans for a home-made squat rack
What more could you want?
Want to know the exact dimensions of that bar you're about to transform? They're all here.
Photos of various home-made equipment.
How-to articles on construction of Hammers and Weight for Height/Distance equipment.
Assorted DIY goodness including photographs of a home-made pulldown attachment and a Reverse Hyper bench for the rack.
Ross Enamait forum - Training Equipment
Notes on the construction of various Highland Games equipment.
Excellent advice on joining materials to other materials.
Construction history of many items in the outdoor Toffe's Gym (which is in a superb forest setting, incidentally) including calf-training equipment, a heavy-duty bench and of course the power rack. Excellent stuff.
After reading those, it's time for a trip to the nearest hardware store. Plenty of fun things to experiment with.