LimeWire is a peer-to-peer file sharing program based on the Gnutella network. Unlike most other Gnutella clients, LimeWire is notable for its ease of use and intelligent client configuration. The user may choose to search in a particular community specified by file type or geographic locations.
LimeWire also attempts to correct the freeloader problem by letting the user choose to prefer nodes that share files and information.
Subjectively, I have found it to be the best alternative to Napster for MP3s and also the best alternative to Scour for media in general.
LimeWire is available at http://www.limewire.com.
Update: Recently discovered that the brilliant programmers who made this could not spell "download" correctly... "Kill downlaod" is how they refer to it.
Update to Update: The new version fixes the typo.