Gracenote (formerly Compact Disc Database)
http://www.gracenote.com/index.html
Gracenote's signature CDDB® Music Recognition Service™ is the industry standard for music recognition services. It has the largest online database of audio CD and song titles in the world and is accessed by over 20,000,000 unique users each month.

CD Cover Central
http://www.cdcovercentral.com
Huge cd covers archive. Great site to find cd covers on the net.

MegaSearch
http://mega-search.net/
MegaSearch searches through over 280,000 cd covers from more than 350 sites.

All Music Guide
http://www.allmusic.com/
The All-Music Guide is a CD guidebook which also has a searchable database on the internet. I use this site extensively and provide links to this site for most of the cds in my collection.

Yahoo! Music
http://music.yahoo.com