I've got a version of Puppy Linux which I can boot from a 128Mb USB key drive. I downloaded the Puppy iso file, (no the latest version BTW)burned the image to a rewritable CD (You can even use the small CDs used in Video cameras as a second bootable device!)
I booted the CD and used the utiity on it to make the bootable USB drive. Job done!
The hardest bit was finding a 128Mb drive which looks like a small pink VW beetle!!