Linux live CD
The Live CD is a way of sampling linux without having to commit to it or changing anything on your computer. If you put a Live CD into your computer and boot from it, you can play with a complete linux system without it touching anything on your hard disk. Examples: Knoppix, Ubuntu Live.
The Guardian's technology section had an article on this way back in November 2004.
Live CDs are not just useful for testing out linux without committing.They can also be used for data recovery and various other purposes. Whole books have been written about the various uses to which Knoppix in particular may be put.
Literally hundreds of different Live CDs are available at http://www.frozentech.com/content/livecd.php. Some are for very specialist purposes, others are for more generic desktop use.