International Music Score Library Project
It could just be the largest virtual library of public domain music scores, or at least the largest one I have ever stumbled upon. The International Music Score Library Project, also known as the Petrucci Music Library after Ottaviano Petrucci (credited with producing the first book of sheet music in 1501), holds over 83,000 scores with 1,500 recordings for over 33,000 works of over 4,700 composers.
So if you u need some music scores, want to learn more about different time periods in music history and its composers, or simply want to download some of the greatest compositions, this site is a gem. And beginning January 22 of 2011 it also started to support video recordings.