
Making wooden handles for your dumbbells. You'll find more great stuff on Tom's article page including notes on the construction of a plate-loading sledgehammer and a modified adjustable gripper.
Brian's Strength Training site
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.
Edgewalls - How to Build a Home Climbing Wall
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
Bill McBride
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
Some great ideas here (start in this thread). If you're hungry for more, take a look at his articles on constructing (and using) a waterball and a sandbag [.pdf, 344kb].
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.