Finding Digital Scans of Sheet Music?

Crymson asks: "I've been trying to find a repository of sheet music out on the web. I'm mostly interested in Classical, although scores for Brass pieces would be nice. I'm sure with Google digitizing all the books of the world, someone must be digitizing all of the sheet music. I don't want special viewers, and I don't want to pay out the nose for music that *may* be what I'm looking for. Where is a decent repository of free sheet music?"