Here is an ongoing list of the best online resources for free sheet music (and recordings) to print and play.

BEGINNERS: 

• makingmusicfun
• 8notes
• musicnotes
• free-scores
• music-scores (lots of duet music, ranging in difficulty)

• imslp
• naxos (University Music Faculties usually pay for student access - ask your uni)
• pianofiles
• pianolicious
• mutopiaproject

These will help teachers, students wanting more, and the self taught musicians. Please comment below if you know of any more!