Just after Christmas, The British Library’s Medieval Manuscripts blog (@blmedieval) tweeted about their catalog of more than 35,000 public domain images of medieval manuscripts. As of 23 November 2012 they had images from over 4,200 different manuscripts!
If you are into the scribal arts, particularly illumination, this is a literal treasure trove of material. Even if, like me, you lack the skills (or patience) for the scribal arts, this catalog is the next best thing to travelling to the British Library yourself. Unless you know what you are looking for, I recommend checking their virtual exhibitions.
I started with the Arthurian manuscripts. What did you start with?