I use a slimmy:
http://www.koyono.com/slimmy/slimmyKata.htm
It holds 1 ATM card, a few $20s and some receipt or transit pass. Basically 3 things - intentionally.
It has an L-shaped stitching so that only the edges peel open slightly, it doesn't unfold. As a result, it's very thin.
It's for your front pocket. I keep my pocket money loose in the other pocket. There's nothing in my backpockets.
I'll never carry a full sized wallet again. No more of that asscheek tumor feeling.