Soulseek is a peer-to-peer file sharing application created by Rosalind and Nir Arbel. Soulseek's original intent was to provide a small P2P music-sharing platform for the IDM mailing list community, but it has grown considerably since, and it now caters to fans of all genres. The focus, however, is still electronica and post-rock. If you're a fan, Soulseek is almost guaranteed to extract a shriek of delight from you as you find that rare warp records import EP you've been searching for.
Another great thing about Soulseek is that the user-base consists mostly of music geeks that prefer high quality MP3s (192kbps and higher) and full albums to creaky broken downloads.
The interface is similar to WinMX and is very easy to figure out. The program itself is fairly stable but unfortunately it guzzles memory the longer you leave it on, so make sure to restart it every so often. It contains several neat features, such as a wish list, a buddy list and genre-specific chat rooms.
The network architecture itself is decentralized. However, Soulseek depends on a centralized hosts file, and there has been some concern that the RIAA would target it eventually. However, since the network is relatively small, the possibility of the network being shut down in the foreseeable future remains remote.
Soulseek does not support multi-threading (downloading the same track from several computers at once to increase download speed), so expect average speeds and potentially long queues. If you're trying to download a Top 40 hit that's stuck in you're head, it's probably not the best platform. But if the phrases "Autechre bootlegs" or "rare Mogwai ep" make you pant with desire, you should definitely check out.
I'll stress again that if you're looking for commercial, mainstream music, you'll most likely be disappointed with Soulseek. It's a small but proudly specialized network.
Finally, Soulseek is spyware-free, adware-free, and cost-free. Check it out at http://www.slsk.org/