Last year, when visiting the new Fry's in Renton, I ran across one of these, and bought it instantly:

Sure, USB drives are always useful, but this has one additional compelling feature: it's small enough to put in your wallet. I find it much more convenient to carry and use than the versions that fit on a keychain. The only downside is that they may be more fragile than the "big" USB drives, but I haven't had any problems in the six months or so I've been carrying them around (I have two). If you buy one, make sure you get the version that comes with the credit-card sized holder.

There's also a USB 2.0 version, reviewed at ExtremeMhz.

Highly recommended.