I've done a good deal of research into filesharing and am familliar with the vast majority of services out there, and have reached some conclusions I hope will be useful to you all:
First off, the best "napster replacement" is
AudioGalaxy. It has by far the best mp3 content of any filesharing service out there, although it is unreliable and often down. BUT (and this is a big but), it installs
Webhancer with itself. Webhancer is a malicious spyware program which sends all urls you visit (in any browser) to them, as well as patches and replaces certain system files related to tcp/ip in order to intercept all your network communications for this purpose. It, like all spyware and adware, can be removed with the free program
LavaSoft AdAware, which will safely and completely remove such malicious software. If you remove Webhancer with AdAware, Audiogalaxy still works.
Kazaa and
Morpheus are very similar applications -- they are peer to peer gnutella derivatives which share files of any type, and which have almost identical interfaces. One great advantage of them over all the currently available gnutella clients is that they have multi-sourced downloading (that is, when you download a file, it downloads parts of it from every user that is sharing the file, so you donwload at the combined speed of all the people sharing it), so you get very fast downloads. Unfortunately, Morpheus is unreliable for me, but most people prefer it over Kazaa. As I understand it, Morpheus searches its own network, and Kazaa searches its own network (separate from the Morpheus network) in addition to searching the Morpheus network, so Kazaa's content = Kazaa+Morpheus while Morpheus's content = Morpheus only. However, Kazaa does not allow mp3s of bitrates higher than 128kbps to be shared, so it's useless for getting good quality music. Both networks have severe problems with content; the only thing that is shared in abundance on Morpheus and Kazaa is porn. Kazaa installs the
Cydoor ad client on your computer, and refuses to run if you remove Cydoor by using AdAware. However, I use
WebWasher to block ads when I'm browsing the web, and since both Kazaa and Morpheus use Internet Explorer to display their interfaces, WebWasher blocks the ads Cydoor delivers, so although I have Cydoor on my system I don't get any ads from it.
WinMX is a formerly great client for Napster in addition to its own gnutella derivative peer to peer network. It was one of the first Napster clients to allow you to connect to many Napster-compatible networks at once, so you were able to get by far the largest library of files available at the time by connecting to dozens of Napster-compatible networks simultaneously with this app. However, largely because of WinMX, all these networks became hopelessly overloaded, and it is now pretty much impossible to get on any one of the servers; now that Napster doesn't run its own servers and MusicCity stopped running its OpenNap servers, the only networks out there are amateur servers run off cablemodems and such. Fortunately, the WinMX Peer to Peer network is generally responsive and high performance, although it has mediocre content compared to the WinMX heyday. It has a reasonable ammount of music, software, movies, and so forth, but is not an excellent choice for anything. If I'm looking for something that's not very rare, this is usually the service I use. Note that in order to connect to the WinMX Peer to Peer network, you need to either not be behind a NAT or forward two ports to yourself.
The famed Gnutella is in a state of disarray, but is still good for finding content that you can't find elsewhere. There is a wide variety of clients for it, the most popular of which is
BearShare. Be forewarned, this is another application that installs spyware, although I believe AdAware can remove it all. It has a good feature set and attempts to improve the responsiveness of the gnutella network by introducing a variety of under-the-hood changes that I won't get into here. I don't use BearShare though; I use
LimeWire instead. Although the interface is a bit clunky, it's backdoor-free and has all the features I need. The array of gnutella clients available is dizzying, and since I only use gnutella as a backup to other filesharing services, I can't get into them here. Note that forwarding the proper ports to your gnutella client will improve the performance of the gnutella network as well as allow you to download from far more clients.
An exciting new service is
eDonkey2000. It allows multi-source downloads and has become popular for the sharing of very large files, in particular full-length movies. It has, far and away, the best content of that type. It has a peculiar network architechture, in which clients connect to servers wich don't connect to each other for searches (although searches can be extended from one to another server), though they do connect for the purposes of finding many sources to download a file from. This makes finding a particular file difficult, but searching for a category of files (e.g. search for avi to find lots of divx movies) is a good way to find a good selection of fast downloads. This is my personal favorite at the moment, since it allows multi-source downloads even from people who are currently downloading the file, so publishing a new file to the network is much faster than any other service. Unfortunately, the content for mp3s isn't very good, since most people prefer to share large files on this network rather than small ones.
DirectConnect is another service that I've heard a lot of good things about, but I haven't had time to test it yet.
Happy filesharing.
-=Marcus