It's exactly what it sounds like: just an index of (apparently) all the period MSS that have been fully digitized and put online -- over 1300 of them. You can slice and dice them by location, language, whatever, but I'm having fun simply browsing the title list. Just a few of the gems I've noticed so far:
- A 15th century French introduction to Cabala;
- The Jeu de Robin et Marion -- apparently a song-story from the Robin Hood cycle;
- Le Livre des Eschec Morales -- which, unless I miss my guess, is probably the French original for the chess book that was the first thing Caxton printed in English;
- An interactive Flash digitization of the Magna Carta;
- A compilation of books from the Grail Cycle;
- A Haggadah from c. 1300;
- The Snorra Edda (in old Norse);
ETA: And while we're at it, here's another collection, albeit tending to be of shorter and more scattered works...