The Petrucci Music Library is a repository of free sheet music scores online. The collection is ever-growing, so far at around 70,000 public domain and composer shared music scores.
Sheet music can be downloaded as a PDF file and printed. The quality can vary, but most scores do have a posted quality rating. You can search or browse by composer, time period, composer nationality and genre. You’ll find a wide range of composers from Bach and Beethoven to Handel.
As well as the scores you can download and listen to user contributed recordings of performance of the scores. You can even upload and share your own!