Working at the Music Library, you tend to build up a personal knowledge base of sources that you like to turn to when you get a specific reference question.
For some reason, I've found that lately we've had a lot of string players requesting scores + parts to things that are already checked out or aren't in print. When this happens, I turn to IMSLP, although now it's called "IMSLP / Petrucci Music Library."
http://www.musipedia.org/ is a searchable database of musical tunes, melodies, and themes. You can play, whistle, or tap your entries! Try it out!