Super bored today so I screwed it together by hand. Not fun, but more fun than sitting on the computer. Got it "done" and hooked up as it started raining. Still needs a top surface and some sealer/paint.
More images:
ipernity: Bike Cargo Trailer by Tyler Linner
I should mention that the only power tool I've used on this is a hand-held electric drill. The only new parts are 4 sq ft of plywood, two wheels/tires, an axle and four nuts. The rest is recycled!
Up to this point if you did it smartly you could probably do it for $15-20 if you used the old wheels.
Holds at least 150 lbs.