The popular LifeHacker website has recently run a small, but useful review of USB booting linux distros –
http://lifehacker.com/5069054/battle-of-the-thumb-drive-linux-systems
Personally I always keep a copy of DSL on a usb key with me – its helped me on numerous occasions (Most recently to get access back on a windows 2K server which had be “re-passworded”!)