I've long wished there were an equivalent of IMDb for music.
You would have songwriters linked to songs linked to recordings (linked to albums and producers) and performances (grouped by concerts and also linked to recordings) with both recordings and performances linked to individual performers and bands (linked to performers).
Can someone please start this? I don't have the time :-)
UPDATE (2005-04-22):
A bunch of people wrote to me and mentioned MusicBrainz (which I knew about) and allmusic (which I didn't). Allmusic definitely seems to be closest to an IMDb for Music although the openness of MusicBrainz appeals and might enable some of the more obscure information (like producer, mixing and mastering engineers) that I'm interested in. Integration with things like the ASCAP ACE Title database would be good too.
Thanks to Michael Plump, Henning Koch, Will Guaraldi, Kevin Dangoor and Gavin Burris for replying.