Damn Small Linux (DSL) is a distro that’s less than 50MB. It’s great for booting off a USB stick (directions here). The problem (or feature) is that by default things don’t persist on DSL. To get that working, you’ll want to periodically back stuff up somewhere– in my case, I wanted to plop it back on the original drive I booted from.
To do that, just right click somewhere on the desktop and go to system > backup/restore. You’ll need to enter in the device that represents your USB stick (probably sda1). This will write a gzipped tar file to the drive. To restore, you just need to supply a “cheat code” of “dsl restore=sda1”. Note that this won’t remove any of the default files already in the home directory (e.g. events.cal) on the restore; it only restores stuff you’ve added or changed.
Also, the USB drive is mounted under /cdrom, so you can manually fish out any additional data you’ve stuck there as well.