Linux

Linux is a free Unix-type operating system originally created by Linus Torvalds with the assistance of developers around the world. Developed under the GNU General Public License , the source code for Linux is freely available to everyone.

There are many versions available for more information please visit DistroWatch another site to look at it http://www.debianhelp.co.uk/tools.htm
A easy way to have a look at Linux to use a live CD/DVD.
To try it you can try Knoppix or even easier go to Ubuntu another distribution of Linux and have free CD's sent to you they will send a install CD and a live CD.
A Live CD can be run directly from the CD or DVD drive, without installing anything so you can safely try it out and once you take the CD out and reboot that’s it you can back to using windows.

Read more at http://www.linux.org/info/