eMule is suffering greatly and has been for the last few years from the lack of decent central servers. Kad keeps ticking along but this is a network in significant decline.
Kazaa? Not been around for years. Visit their homepage: "We thank you for your interest in Kazaa. However we no longer offer a music service."
Older versions of Limewire still connect to Gnutella but the network is unusable. Full of viruses and malware and very little decent content. One thing the LimeWire devs did manage was to keep the network fairly free of spam.
Frostwire - as someone else mentioned - is now a bittorrent client only and if you want a bittorrent client, you're better off with uTorrent or something else.
Gnutella - see above.
Freenet? Seriously? Yes, it's still there. Yes you may still be able to download a few things on there but it has the same issues it always did. Slow. Poor availability. You have to denote a portion of your own hard drive to store who knows what (but possibly encrypted kiddie porn).
So how do people get music now if they're not using legitimate methods? BitTorrent. MP3 download sites. Stream or rip off YouTube. But over the last two years, the legitimate options have become - for the first time ever - better than the piracy experience. There's stuff on Spotify I've never found on a single piracy site bar Oink and VK and the first of those is dead (though What.cd is a good successor) and the second Russian which means it just doesn't get considered by those in the West.
It never went away, it has always been my number one choice for music downloading, whether it's incredibly rare releases, or the most commercial stuff I use for my DJ'ing.
Put simply, there's very little music you can't find on Soulseek, especially when you use the "wish list" function and have the program auto search for you at regular intervals.
Soulseek is characterised by having a lot of music devotees. These are people sharing because they love music, not just because they happen to be downloading whatever the hell it is that's television series of the month. So you've got a committed userbase with a wide range of content and so yes, it's much better if you want music.
