FreeDB: an open-source plug-compatible CDDB replacement
FreeDB.org is one of the OtherDatabases of music meta information; it is the open version of CDDB. It was formed in response to the actions of Escient, which through its subsidiary Gracenote, bought the rights to CDDB and restricted access to the data.
FreeDB took the last available snapshot of data, and created their own server and database. Technologically, it has remained fairly static since then, but it has amassed a very large number of releases. The quality of the data is not always the best, and there is a lot of duplication in the FreeDB database.
There is a web interface to FreeDB but most access (lookups, submissions) takes place via the FreeDB protocol (which may be embedded in HTTP, but is not really accessible via a browser). One of the advantages of this protocol is that it is practically the same as the original CDDB protocol, and could thus be used as a drop-in replacement for CDDB when Gracenote took it over.
As a result, FreeDB has taken over as the de-facto standard CD recognition database for open souce music software. MusicBrainz has many features included to import data from FreeDB. Eventually, it is hoped that MusicBrainz will take over from FreeDB as the preferred open-source free-database way to look up CDs, especially since Gracenote has since developed (and is forcing customers to switch to) CDDB2, a proprietary replacement for the original CDDB protocol.
Note that there is a FreeDBGateway, which allows you to request MusicBrainz data through the CDDB protocol.
You must carefully examine the FreeDB data since it needs to be corrected in most cases to confirm with the MusicBrainz StyleGuidelines. Also, it is good practice to provide proof of your release in the edit notes, FreeDB is not considered as a proof, as their submissions process is not monitored.