Kazaa
Kazaa is a file sharing program that allows you to search and download audio, video, images, documents and software files. Kazaa has had problems with people contacting viruses from their software but Kazaa are trying to sort the problem out and have a FAQ to help users avoid being infected. Also, Kazaa Media Desktop includes various Spyware software which may turn your computer into a remotly controlled node. Alternatives are Kazaa Plus, which isn't free (it costs around $30) and may also include Spyware, and Kazaa Lite, which has been declared illegal by Shareman Networks (the Kazaa developers). Be aware that, in addition to al the above, Kazaa is only available for Windows 9x/Me/2k/XP platforms.
Kazaa Media Desktop homepage - http://www.kazaa.com/