In short, The Mutopia Project is a collection of more than 1,400 pieces of sheet music (with more coming all the time) that are free for you to download, print, perform, record, or even create your own editions. Put it this way: if you can find what you want in Mutopia, you will never need to go into a music store again.
They could have just as easily called it "Music To Your Wallet." But I don't suppose "Muwallet" makes any sense.
Using Mutopia is very simple. You can browse by composer, instrument group, style, or selected compilations. Or, you can use the handy little search box and search for a piece by name.
When you get to the piece you're looking for, you'll see a table, which contains links by which you can download scores in letter or A4 size, midis, or click to various kinds of additional information. Download is super easy; click on the letter or A4 links and the score loads in Adobe Reader. Print, and there you are!