The International Music Score Library Project (IMSLP) is back online after a hiatus beginning in early 2008. Read about the reopening here. IMSLP (now also known as the Petrucci Music Library) is a wiki for public domain music scores.
IMSLP operates in Canada where a score is in the public domain if it was published more than 50 years ago and the date of death of the last author of the publication also died more than 50 years ago. Consequently, the collection of scores skews toward older pieces. But the site also includes information on how to submit your own Creative Commons licensed scores, so I’ll look forward to watching the contemporary selections grow.
Welcome back, IMSLP!