Many of the British Library's rare or unique 16th-century music editions are now freely available online, thanks to a partnership between Royal Holloway, University of London, the British Library and JISC.
The Early Music Online project has digitised from microfilm more than 320 anthologies of printed music from the 16th century. The earliest, a collection printed by Ottaviano Petrucci, dates from 1503. Highlights of the collection include sacred music by Josquin des Prez, Thomas Tallis and William Byrd; secular songs from Nuremberg, Paris and Lyon; lute music from Venice and organ music from Leipzig. Over 9000 individual compositions have been digitised.
You can access these digitised editions free of charge via http://www.earlymusiconline.org