We've covered this particular freebie in the past, but it's too good not to share again every now and then. If you play a musical instrument, make sure you check out IMSLP.org, also known as the Petrucci Music Library for free music scores and sheet music. It's a huge virtual library of public domain music.
There are scores and sheet music for classical music, opera, non operatic-vocals, and more! In total, there now are over 230,000 scores that are completely free. The site also has music books and recordings of music available.
Whether you're a student, teacher, or just a music lover, this is a great resource. Make sure you check it out!