I recently took up piano and I was surprised (read: shocked) not to find easily downloadable music partitions (sheets) on the internet. Now I am not looking for partitions that would be copyrighted, say, by Alicia Keys. I'm more hunting for public domain compositions by classical composers like Mozart, Bach or even Gershwin.

So is there a place on-line that allows to download public-domain partitions for classical (symphonic) music?