With all the controversy surrounding Napster, and being only limited to searching for MP3's -- the next step is Gnutella. Originally developed by Gnullsoft, their parent company (AOL) pulled the plug on this piece of software.
Now, development has continued by an anonymous source...
What is it? -- Gnutella takes over where Napster left off. It allows clients to search for all types of files -- not just MP3s. It also avoids a "feature" of Napster. Napster indexes all clients on a centralized server... as a result, that site becomes a target for unhappy companies.
Gnutella users client-to-client searches, which decentralizes searches. It's like a big game of "telephone". Your system asks the next system, which then asks the next and so on.
Originally Windows only, there is now a version which works on the Mac. It isn't native yet, and only runs under Java... but the required files are listed on the site.
We here at macrumors haven't gotten around to downloading/install all the components yet, but plan to soon.