Here is an unsolicited review of a very helpful music site. I recently was looking for some good sheet music for my students and I found www.makingmusicfun.net. This site not only has some fun arcade style games you can play, but a wealth of resources and some of the best sheet music for students that I have found!
Sometimes one of the best motivations a student can have is a great arrangement of a song they have always wanted to learn. While this is not the single most extensive collection of sheet music, it is a very useful, well organized collection of good, solid material. I found the site easy to navigate, the music easy to open (pdf’s), and the levels well marked. I am currently printing out some for my students that I know they have been wanting to play.