As Multi mentioned,
ANts and
MUTE are open networks designed to provide IP number anonymity with a smart, organically inspired routing method. Also
Filetopia is worth mentioning here. Their approach is more straightforward, based on anonymizing middleman (proxy) nodes that they call 'bouncers'.
It's hard to rate the anonymity quality of these programs as their stealth capabilities depend on factors like the number of people in the network, the size of files being shared and of course the type of attacks (see for e.g.
this article). MUTE and ANts are probably your best choices anyway today if you want to remain anonymous while filesharing - the downside is that these networks don't have yet much content nor are they fast.
- tg