The site is The Mutopia Project. The site offers sheet music for nearly 1500 compositions.
Most of the pieces are classical and in the public domain. They include works by Bach, Beethoven, Chopin, Handel and Mozart.
But they are adding more modern composers as well, by making arrangements with their editors, arrangers and composers.
The site has a search engine, where you search by instrument or composer. When you find the piece for which you're looking, simply follow the steps on the screen.
I had to get Scott Joplin's "Entertainer" for my son's piano lessons. Instead of paying several dollars for the sheet music (plus gas or shipping), I was able to print it out FOR FREE!
Check it out!