If you have any interest at all in Gregorian chant, then you must MUST visit this site: Gregorian chant books for the Roman Catholic liturgy. It’s the best source that I’ve seen anywhere that pulls together not only the massive corpus of chant materials but links to where such things can be found on the web.
In terms of medieval material, the editor only skims the top, pointing out the key manuscripts of the San Gall library and does not connect to the many other chant manuscripts in many other digitized collections around the web, but this does not at all diminish the vast volume of work that has gone into this.
They say that to the man who has a hammer the whole world looks like a nail. I’m more than guilty of this myself, but—as a database guy—I can’t help thinking that supplementing this long tabular format with a basic database interface for easy access would make it that much more useful.