Here’s what they say about themselves:
Welcome to the portal page of the International Music Score Library Project (IMSLP)! We at the IMSLP believe that music should be something that is easily accessible for everyone. For this purpose we have created a music library to provide music scores free of charge to anyone with internet access, with several other projects in planning. IMSLP is also entirely collaborative, and all contributions are greatly welcome.
You can browse by composer (aphabetical), composer (time period), instrumentation, genre, recordings by composer. As of March 15, they have 88,000 scores and 5,000 composers.
There’s also a NY Times article here.
