Sheet music is very hard to find on the internet (if you don't want to pay for it at least), so I decided to share what little I have. I put the pictures of the sheets up to keep it simple, piano people aren't always familiar with stuff like zip files as I read on some forums :) Click on the small pictures below to see a sheet in full, then right click on it and select "Save Picture As..." to save it or "Print Picture..." to print it.
If you're prepared to pay for sheet music, this site is probably a good place. It's strange that the music of the classic composers is still not available for free though; classic literature is easy enough to find online.
Gary Jules - Mad World
This song was originally made by Tears for Fears (an 80s pop band for you young people), but Gary Jules made a superb new version for the (also superb) movie Donnie Darko. In my webstats I noticed that a lot of people (about one each day) who are looking for the sheet music (partituur; English needs a proper word for that!) of this song end up on my humble site because I mentioned it among my favourite songs.
So I went in search of it myself and found it on Mithrandir's Sheet Music Page. This guy Mithrandir wrote down the score himself and it seems pretty accurate. On his page there's a link to a music program called Finale Notepad, and at the bottom (under "Miscellaneous") you can download the file with the score of Mad World. If you load this file into Finale Notepad you can view the score and play it. Here are pictures of the five pages of the score.
J.S. Bach - Toccata & Fugue in D Minor
I bought an electronic piano and started learning to play it mostly because I want to be able to play this fantastic organ piece one day. It'll take a few more years but I can do the first two pages already, which is a lot of fun especially when I connect my piano to my stereo :) It's probably helpful to hear this while you read it; you can play this awful midi version for example.
Ludwig van Beethoven - Mondschein Sonata
My favourite piece of piano music. I wonder how you're supposed to play this with only two hands though.