I’ve been on a project at work that involves tracking down information on some music scores, and have encountered a fantastic resource, the IMSLP/Petrucci Music Library.
On this mediawiki-based site resides more than 72,000 scores representing over 29,000 works by over 4,000 composers. They even have over 1,000 recordings. All of this is available for download, as they focus on works that either exist in the Public Domain or have been made freely available by the rights holder (though one should note that the rules for Public Domain vary by country).
More information can be found on the International Music Score Library Project’s main page, including links to browse categories and a forum site.