The sources archived include all the fragmentary sources of polyphony up to 1550 in the UK (and almost all of these are available for study through this website); all the ‘complete’ manuscripts in the UK; a small number of important representative manuscripts from continental Europe; a significant portion of fragments 1300-1450 from Belgium, France, Italy, Germany and Spain.Obviously a great resource for scholars (they've started to include images taken under ultraviolet light as well), but also surprisingly fun to just browse around. It takes some time—the search functions are geared towards experts trying to pinpoint particular manuscripts or collections—but there's plenty of graphic treasure to be found. Here's a few favorites:
From Oxford's Bodleian Library (MS. Don. b. 31), an Agnus Dei with the incipit beautifully encased in the initial Q of "qui."
Three successive initials on page 54 (verso) of MS 178 from the Eton College Library. I love how the obviously Irish scribe indulges himself on that third one.
And from the British Library (Egerton 3307), a Medieval house party. Even the dogs and monkeys are getting drunk. Rock-band excess has nothing on the fifteenth century.