Creating a bootable version of Windows, Mac OS, or Linux is a great way to repurpose your old USB flash drives. You never know when you might need to reinstall your favorite operating system or repair the PC of a family member or friend, so take a moment to read over our guide to creating bootable flash drives--and then build a portable PC toolkit you can carry in your pocket.
Diehard Windows users will be happy to know that Microsoft makes it easy to download a copy of the Windows 8 consumer preview and stick it on a flash drive for installation on any PC, anywhere. Check out Rick Broida's guide to installing Windows 8 on a flash drive if you're interested in throwing the Windows 8 Consumer Preview onto a USB stick that you can give to family and friends eager to play around with the future of Windows. If you want to keep a copy of the current version of Windows handy in case of emergencies, you can download a copy of Windows 7 to your flash drive and use it to install Windows 7 without the disk. The trick to installing any version of Windows from a flash drive is to download the Windows 7 USB/DVD Download Tool, which will allow you to mount the .iso image of Windows 7 or 8 on your flash drive and boot from it as though you were booting from your Windows installation disc.
If you'd rather stash a spare Lion in your pocket, check out David Daw's guide to creating a bootable Mac OS X flash drive. One of the best-kept secrets of Mac OS X Lion is that you can now install a bootable copy of Lion directly to a flash drive, giving you a portable, fully functioning version of the operating system. Even if you don't use a Mac OS X PC at home, having a bootable version on your keychain can be invaluable when your friends and family ask you for help troubleshooting their favorite Mac notebook or desktop. Just make sure to use a flash drive with a capacity of at least 4GB; otherwise, your captive copy of Lion won't fit in its cage.
Last but never least, Linux fans should check out our guide to creating a bootable Linux flash drive. In just three simple steps, you can park the latest version of Linux on a flash drive that you can carry on your keychain; and since Linux is open-source, it's easy to customize your Linux distro to suit your particular needs. Even if you don't use Linux on a daily basis, having a stripped-down copy of the OS available in case you need to boot into a damaged or malware-riddled PC (and you can't rely on Windows or Mac OS X to function safely) can be very handy.
In fact if you have a few spare flash drives lying around, it's a good idea to convert one into a bootable maintenance toolkit that you can use to diagnose and treat a malfunctioning PC. Load a USB flash drive with a copy of your favorite OS and a few maintenance programs such as Revo Uninstaller Portable and Recuva Portable, and you'll be able to wipe and reinstall operating systems, recover files, and uninstall unnecessary programs with a single flash drive that's compact enough to fit in your pocket.