The International Music Score Library Project (IMSLP) has reopened! The IMSLP is a huge database of musical scores that have become public domain. It currently holds more than 16000 musical scores. Although some of the scores are a bit dated, the IMSLP is a great resource to find that music score of that famous composer you always wanted to play. But there are numerous lesser known composers in the library as well, which makes the IMSLP a great place to discover hidden treasures in music.
Just to give a simple example of the vastness of the library, have a look at the Beethoven page! Or the Mozart page! Now that'll keep you busy for a while... This is a highly recommended resource for any musician or student!