Limewire is probably the best-known Gnutella file sharing client. It's a Java application, so it's cross platform. It provides a built-in audio player and library, and also integrates with iTunes. It has an optional content filtering system that works with copyright owners to help prevent items that aren't legal to share from being uploaded or downloaded. There's also a built-in chat client so you can send text chats to others running Limewire (although not other Gnutella clients).
Unfortunately, Limewire's interface, although improved from previous versions, still borders on ugly and is definitely clunky. It has a decent range of features, but it isn't pleasant to use. It also exhibits various Windows-oriented options, such as the System Tray item in its Preferences list. Although it's widely considered to be the granddaddy of the Gnutella file sharing system, we really can't recommend it.
