Kazaa Lite and Imesh are the 2 biggest. IRC's are the most dangerous and most tedious to use. Newsgroups seem to be real popular outside the USA but I don't know anyone in the USA using them.
Now the 2 warnings.
1) It is now common practice for people to load trojans, worms and viruses into P2P network posts. I have 2 friends who had to reformat and reinstall their systems.
2) The RIAA and movie studios have hired companies and hackers to monitor P2P traffic for copyrighted material. If you have any on your computer and are sharing it they will find you and try to have your ISP shutoff your service. Most ISP's are resisting this pressure pending legal challenges to the DMCA. If you have a wimpy, gutless, ISP they will shut off your connection for sharing copyrighted material. They just nailed a friend of mine in the time between when the file finished downloading loading and when he got up the next morning. It took him 4 days to beg his way into getting his DSL connection turned back on.
Try before you buy is the only way to go with the junk software being sold by people like MS, Sierra, etc. Just be aware of the risks you are taking.