If you look long and hard enough you can find Firewire thumb drives, Micromat uses one for their Protege
. But those puppies are not exactly cheap and their capacity is very limited. Micromat makes do with a special stripped down version of OS X on the Protege.
Knoodle's solution is the one I arrived at several months ago. I was upgrading the 160GB hard drive in a friend's older MacBook Pro with a shiny new 500GB drive. As my fee for the upgrade I kept the 160GB drive and mounted it in an OWC Mercury On The Go enclosure. Formatted it, installed OS X and a variety of disk and system repair utilities and it is now my general purpose Emergency
drive. With the help of a $15 powerbrick it will even boot my MacBook Air so I can run diagnostics and disk repairs if necessary.