Linux
I'm a big 
Linux[0] fan. Linux is an operating system originally developped by Linus Torvalds. Now many programmers around the world develop Linux. The source code is freely available. But still, the last decisions on what comes with the standard kernel (the base of a Linux operating system) takes Linus Torvalds.
Many distributors around the world take this kernel and other (free) software, make packages out of these things and sell these distributions for a relatively low price or make them available for download.
Some time ago, it was quite difficult to install Linux and work with it. Nowadays this has changed a lot. Distributors try to make it as easy as it can be. New hardware-support is added every day, so you can use Linux with most modern computers. Notebooks, though, are still a bit more difficult quite often.
I have to admit that i don't know many distributions yet. I started with SuSE Linux. I don't like them any more so at the moment I'm getting familiar with GNU/Debian Linux. Debian's really nice... a bit tricky now and then but great fun and good to work with! For some situations I also use Red Hat Linux. They don't sell any personal packages any more. The personal packages can be downloaded as Fedora Linux now.
If you want to try out Linux without having to delete anything and without installing anything you could do that by using Knoppix, which boots from CD.
Another distribution which boots from CD is grml. It's fascinating, the emphasis with all the text tools is on expert users, though.
If you're interested, have a look at some tips for programs I use!
[0] Linux is a trademark of Linus Torvalds.
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Last update: December 13, 2004
(c) 2001-2008 Florian Fankhauser, http://www.flof.at/