Oink's Pink Palace. Yes it was a torrent tracker so arguably illegal, but the breadth and depth of music it cataloged (not to speak of the community) is still unmatched.
Here's a bit of history that might get lost: OiNK's rules specified minimum bitrate of 192k for MP3 uploads. OiNK was a British tracker, and Radiohead's "pay what you want" MP3 release of _In Rainbows_ (2007) was purposefully 160k to be under that limit. I don't remember if the upload was allowed or not :p
