Grokster - A must have if you are into getting anything and everything for free, this is the best P2P of all, there is usually 3-3.5 million users at any given time, sharing 4,500 terabytes ( for those of you who don't know that's a lot of data). Grokster users also have the benefit of searching other users like kazza, fileshare, bearshare, etc. Download this file: cdclint.dll, if you don't want pop-ups. For installation directions click here.
Kazzalite - A good P2P App comparable to Grokster, but is limited to searches within users that use kazza only.
Imesh - Is also a very good P2P, the only con is that isn't very popular yet so the chance of finding what you want is slim, a huge pro is that the searches are extremely fast compared to any other P2P I've seen.
WinMx - Pro: Billions of users, Cons:Very Slow search results, not enough high speed internet users; making downloads incredibly slow.