The finished vehicle. Click to enlarge.
UPDATE: Detailed plans coming soon!
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I built this go-kart soon after learning to weld, and it has certainly been a lot of fun. It reaches a top speed of 45 mph, which is rather exciting since you're just three inches off the ground.
The go-kart is powered by a 5HP Briggs & Stratton Horizontal Shaft Engine which drives the rear "live" axle. A centrifugal clutch lets the engine idle without driving the wheels. When you increase the throttle, it engages the drive shaft. The disk brakes are hydraulically operated from an off-the-shelf go-kart brake kit. In my opinion, hydraulic brakes are definitely worthwhile and require little to no maintenance once if properly set up.
The back of the gokart showing engine and drive axle. The centrifugal clutch is visible on the left side of the engine, connected via chain to the drive sprocket. The disc brake is on the ride side of the drive axle.
As for the chassis, I built it from regular 1" square steel tubing I got from a local steel company at less than a dollar a foot. The wheels are a home-depot specialty: designed for hand trucks. I picked them up for about $10/piece. Several other odds and ends I ordered online, such as the steering wheel, bearings, rear axle, etc. But in general, it's mostly built from common stuff and is definitely something you can do in your garage in 6 or 7 weekends.
The throttle is mounted to the seat, and is operated by the right hand. The brake is operated by the left foot. Feels a little strange at first, but ends up being pretty useful since you can use your right foot to brace against the cart during sharp turns and heavy braking maneuvers.