If you are looking to play pop music, http://www.musicnotes.com/ is a great source. However, you do have to buy the music (have to compensate the writers for use of their work), it's usually USD3-6. They have music for many instruments and you can transpose the piece on the website. For songs with lyrics, there are lyrics, the melody, and the accompaniment.

They do have some free music for holidays and stuff that is in the public domain, which I believe you can download without an account.