I am using SliTaz for a while now. It is a tiny Linux distro which you can run from the USB. It is possible to save the changes, downloaded files as well as to preserve the rootfs (root filesystem) once you are done with installing your regular apps. The modified rootfs is a snapshot to go back to if something goes wrong. It supports all the regular apps like firefox, pidgin, smplayer, flash, gcc etc. Has its own package manager. The distro is extremely fast as it runs off the RAM and has a very small footprint. Can be run on low-end systems without a glitch. Easy to use for end-users (who are advised to used the released version) and a hacker’s delight (who are bound to use the cooking version). Various releases and flavours are available here.