If you are a musician - pianist, guitarist, etc., and you'd like to add some new sheet music to your musical repertoire, you can do this pretty easily with a simple search string. First, let's use Google. Here's what your search should look like:
beethoven "moonlight sonata" filetype:pdf
Okay! Let's break this down. First, I'm looking for works by Beethoven. Secondly, I've got a specific work in mind, and I've got it in quotes so Google knows I want those words in the exact order and proximity that I've typed them in. Third, the "filetype" syntax tells Google that I don't want any results except those that are formatted in a PDF, which is what much of the sheet music out there is written into.
How about Yahoo? Same song, a little bit different verse:
filetype:pdf "beethoven" "moonlight sonata"
Yahoo brings back similar results, with a very similarly worded search string. Remember to put those quotes around the song title you're looking for, it makes a big difference.
One more search engine to try for today: Bing.
filetype:pdf beethoven "moonlight sonata"
Bing's results differ slightly from Google and Yahoo; they're all drawing on different indexes (see Do Search Engines Search the Entire Web? for more on this subject) so it's definitely worthwhile for you to try your sheet music at different sources.
As you search, do a little experimenting with the name of the songs as well as the artist. See if there might be different file types out there that could contain the sheet music you're looking for; for example, many sheet music is uploaded as a .jpg file. Simply substitute "jpg" for "pdf" and you've got a whole new realm of possible results.