Strength / Weight Training

Deep squats are good for the knees if you do it properly. Builds up the muscles that protect the joint. Partial squats, apart from being wussy, actually put much more strain on the knees because you have to use that part of the leg to stop the descent and push back, rather than bouncing out of the squat.

Bouncing is cheating though ;-)

But yeah, you're right - bad form will wreck your knees but good form should protect them.