Windows: Can't decide whether you should commit your spare thumb drive to Windows recovery, Ubuntu or some other live-booting OS? Run MultiBootISOs on your USB drive, and you won't have to choose — you'll just choose between them at start-up.
The folks at PenDriveLinux.com have created a Windows tool for creating what is basically a GRUB multi-OS bootloader on your USB drive, along with a nice long list of systems that work with it. All you have to do is grab an ISO file of the system you're adding to your drive, then drop the whole ISO file onto the drive (occasionally with special instructions). There's all the usual Linux cohorts — Ubuntu, Fedora, openSUSE, etc — but also some great rescue tools and utilities, like the GParted partition disc, Ophcrack password reset and virus scanners. You could even load a Windows 7 installation DVD onto your thumb drive, if you had the room.
I tried out PenDriveLinux with Damn Small Linux, Ubuntu and Gparted on one USB drive, and it worked like a charm. If you find trouble booting up, check out the page text for tips on creating a Master Boot Record on your thumb drive, and fixing other problems that can crop up.
MultiBootISOs is a free download for Windows systems, and we're definitely thinking about going back and adding it to the top 10 USB thumb drive tricks we just finished off.
Boot Multiple ISO from USB (MultiBoot USB) [USB Pen Drive Linux]