I've been involved with this one sight that is focused on a "cult-like" following of people that use kettlebells. If you don't know what they are, go
here. But aside from that, the guy who brought them to the US from Russia is also an amazing trainer. I've used his "Naked Warrior" book of bodyweight training to help rehab my knee after I tore my ACL last year. He uses two main exercises; one called the "Pistol," which is a form of a one legged squat, and the One-Arm Pushup. I was surprised how good my legs and whole body felt afterwards. They require great stability which builds your core and definitely builds functional strength.
He uses this formula called "Grease the Groove" which is basically doing sets of about 5 many times throughout the day. Proper form and tension is an aboslute must and focus is necessary. My legs are more stable now than before my injury. If anyone is interested further I'd be more than happy to get into it a little deeper.
*I am not affiliated with any of these companies, just trying to help ppl out b/c it sure as hell helped me out a lot.