Tyler pointed me at an electronics ad for some really cheap, really tiny USB keys, so I picked up a couple today. They were 128MB, $9.99 each, and are about the size of a penny:

128MB isn't much nowadays, but it's more than enough to store SSH and GPG keys, or maybe a copy of GTD TiddlyWiki. I'm sure I'll find other uses for them too, but hey, if I don't, they were only $10 each.