I know of three: Audiogalaxy.com, Kazaa.com, and Imesh.com
so far i like kazaa the best. nothing will ever have the widespread collection that napster had, but kazaa is nice and simple. with audiogalaxy you have to monitor your downloads through this annoying satellite icon in your system tray.
I use KaZaA and it is fairly straightforward to use. It also cross-searches into MusicCity when looking for downloads.
The major problem with KaZaA (though they would call it a feature) is that if it finds multiple users with the same file (size, bitrate, etc, etc) and you have available downstream bandwidth, it will try to split the download across the users. The problem is occasionally you will have 2 people that ripped the same using the same settings except, for example, one used normalization on the encoding. The result is the recording level of the song will start changing as you hit the different sections of the file downloaded from the different persons.
Of course, KaZaA is spyware (meaning they have compiled in a third party advertising library), and it also is supposed to limit MP3s to a bitrate of 128 or less.
I have to vote for WinMX. I have found every song I have ever looked for, even obscure outdated titles. It is very Napster like with the only real down side is that your hotlist is only temporary. If you are used to Napster then WinMX has almost no learning curve. My 62 year old mother was crushed when Napster took a dump, I told her about WinMX and the next day my Dad couldn't get her off the PC, and she is really computer challenged.