If you play a musical instrument then check out this web site, which has thousands of pieces available for download in sheet music format. Because all of the pieces are copyright-free, this means that there's a distinct lack of modern entries. But it's still a great, comprehensive archive which will be of use to any serious musician.
Downloads are mostly in PDF format, ready for you to print and play.
Just go to http://imslp.org/wiki/Category:Composers to start browsing.
Comments
For choral and vocal music, try this one, over 25,000 scores from over 2,800 composers!
http://www0.cpdl.org/wiki/index.php/Main_Page
Great find, Rob! Here is another free sheet music site your readers may be interested in: http://www.mutopiaproject.org/