TRX fitness is a new method of suspension training that has become popular in recent years. It focuses on using your own bodyweight as resistance, as opposed to free-weights (dumbbells, kettle bells, barbells, etc.).
TRX is great for working your stabilizer muscles. Doing a push-up is pretty easy, but when your hands are suspended on ropes you really need a stable core to do it.
Adjusting the exercises for your own strength level, or even your fatigue level, is also easy. Simply change the position of your feet or hands to make each exercise harder or easier – no mucking with weight plates.