ab roller
I've used a cheap ab roller on and off for a couple years. I started after watching a Jackie Chan movie (Gorgeous) where he uses it from standing up. I'm not sure exactly what kind of muscles you'll get, but after doing three or four reps from standing (start with it at your feet, go all the way out, then come back without stopping), my abs burn for a good 2 or 3 days. It's great for back and arms as well. I wouldn't use it exclusively, but since you can get them for about 10 bucks now, I'd say try one out.
I don't like other ab devices because they isolate things too much. There are plenty of ab exercises you can do without machines.
Cheers