I've been playing around with something kind of cool: flash drive distros. The idea is this: you download a Linux distribution to your computer. Then you write the file to a flash drive. There are tools both in Linux (Ubuntu's Startup Disk Creator, for instance, which comes standard in any current Ubuntu release), and tools that run on both Linux and Windows, such as liveusb-creator.
After that, you pop in the flash drive, reboot your computer FROM the flash drive, and you have a different operating system. Don't like it? Reboot back into your original system, and copy a new distribution onto the flash drive.
The truth is, you can accomplish the same thing, probably even more easily, by using virtualization. But the flash drive approach also lets you carry around the operating system - and some space for your own files - almost anywhere. So you can use someone else's machine and have total security and privacy, without having to carry a laptop around.
This is part, of course, of the general trend of whole computers getting smaller and smaller, a trend whose full impact has yet to be felt.
To date, I've played with Ubuntu variants Linux Mint, and Peppermint OS. I hope to try Fedora soon. Usually, I try them on my HP PC, then plug the flash drive into my Acer Aspire netbook. It's a harmless enough way to spend a day. So far, the Ubuntu Netbook Remix is the prettiest and most interesting. (It didn't work very well on my PC, though.)