When I was searching around for sheet music to use for the Piece Together Play Together activity at our September Surprise a couple weeks ago, I came across a couple of great sites:
8notes.com has done a complete overhaul of their site since the last time I visited (which was quite a while ago!). They have over 500 free piano pieces in their collection, and dozens more exclusively for members. Even if they don’t have the piece on their site, they have a nice list of links to external sites that can save you a lot of time searching on your own. This appears to be a member-driven site, so the quality of arrangements you will find varies, but it’s definitely a great go-to place for free sheet music!
Easy Sheet Music has a collection of over 70 pages of well-known Classical favorites. You can access up to three a day for free. Or you can subscribe for $15/year for unlimited access.