Any work written before 1923 is public domain. That is all classical, romantic, Baroque, and renaissance music as well as the first 22 years of the 20th Century.
In the free music links you will find many websites that have free music that are copies of these original scores. Sadly, sometimes these scans are less than legible.
When someone makes their own edition of a public domain piece of music, they can restrict it, and sell it. I would like to see all major classical works with free, clean, online editions free for download. This is 2012 people.
I would like to start occasionally releasing free editions of some classical pieces on this website. If you would like to send me your cleanly edited editions of classical works that are in public domain, contact me under the contact tab.