Kazaa is the best thing since sliced bread. It's pretty much like napster, but instead of only audio, it includes the sharing and transfer of video, images, documents, and software as well.
The interface and toolset of the client aren't as smooth as napster's, but it's an amazing piece of technology. The beauty of kazaa is that for the first time, bandwidth is now treated as a P2P commodity to share. Files that are available on more than one server are downloaded in parts, simultaneously, from multiple locations. This means the more popular a file, the more likely there is to be multiple locations, resulting in easier downloads. This also means very large popular files are downloaded faster than you could ever obtain them from a single server. Yesterday I downloaded a 110Mb file in about 15 minutes, thanks to the 8 people that all offered full bandwidth to obtain them.
It's just incredible... at pirating music, movies, and software.
(when installing kazaa, be sure you deselect the option to include the HotText plugin, I've heard that apparently it's the spawn of satan and impossible to remove from your computer once enabled)