My husband has been having some issues with his knee, and his physical therapist recommended he try foam rolling. So I got him a big piece of foam exactly like the one in the picture (from amazon) for Christmas. It looks like an over sized pool noodle, only it's harder, thicker around, and rigid. He feels it's really helping him and has been looking up all sorts of ways to roll various muscles on YouTube. When my middle back was tightening up, he was able to instruct me on some rolling points to help alleviate my discomfort. It does feel kind of nice to be honest.
While I've been doing the P90X lean schedule (currently finishing up week 6), I deviated today by joining my husband in doing the plyometric workout, which I'd never attempted before. Some of the plyometric exercises are in the core synergistic and cardio workout and if I could rename it I'd call it "Jump, jump, jump, x infinity." Tony Horton calls plyometric "the mother of all workouts." Indeed, this workout generated the greatest amount of sweat I've experienced while doing a P90X workout.