I have been using the IMSLP.com recently, the International Music Score Library Project. Thousands of scores (85,000 and counting) of classical music have been scanned and cataloged and are available for download. A disclaimer makes me acknowledge that I'm aware of copyright issues; it makes no claim that these are copy-right-free documents.
The project is described in today's Times.
I used it most recently to remind myself of the fourth movement of Beethoven's string quartet, opus 18, no 1, for a review I was writing for ClevelandClassical.com. What a great tool! Does it rip off the publisher? I don't think so: I'm generally of the opinion that the more we have out there, the more we'll want out there: that is, the more is available for free study and practice and, yes, even performance (the Borromeo Quartet uses digital scores), the more demand will increase for printed scores and parts.
What do you think?