Google launched a new music search service today, very cleverly called Google Music. From the Google Blog:
A few of us decided to try to make the information you get for these searches even better, so we created a music search feature. Now you can search for a popular artist name, like the Beatles or the Pixies, and often Google will show some information about that artist, like cover art, reviews, and links to stores where you can download the track or buy a CD via a link at the top of your web search results page.
I just gave it a quick try, and I rather like it! I have long wanted to find a service like IMDB but for music, and I have never come across it. Google Music is most definitely not that service, but it is a step in the right direction. Usually I end up trying to find track listings for albums, and unfortunately Amazon doesn’t always have the track listing. I’ll have to play with Google Music a little more to see if it is any more reliable.
I really like that Google Music will show lyrics for a song, other versions of the song, and the authors too (whoever wrote the song). You can also buy songs directly, currently there are links to iTunes and Real Rhapsody. Would be cool to have a “similar artists” feature or even just “similar albums” so you could discover new music using the search too.
Read: Google Music Search
MasterMaq's Blog 
I haven’t tried it yet, but the most comprehensive repository of music info I’ve been able to find is the All Music Guide. It’s got trustworthy ratings, lists only the correct tracklistings and also lists all the personnel that did work on the album.