File Sharing

Kazaa, with over 100,000,000 downloads, is certainly the new leader after Napster. Kazaa's growth is due to its variety of content (which ranges from music to videos, software to images, and documents), simplistic nature, and built-in full feature media player.

Download speeds are certainly sometimes slow, especially for my high speed broadband connection. And another downside - if you don't have a lot of memory, multiple large video playbacks w/o restarting is a strain.

New and improved versions come out all the time.

Cons:
sometimes slow downloads for broadband connections
large file playback can be too much for some computers

Runners Up:

2.

3.

4.