The Micro Vault by Sony are the smallest that I've seen but I use the Lexar Firefly and I think it is a bit more rugged. I can fit the Firefly and the Sport together in the fifth pocket of my jeans.
You might want to check out this thumbdrive:
http://www.engadget.com/2007/03/15/kingmax-unveils-worlds-smallest-usb-flash-drive/