Among the first-generation digitization projects in the humanities is disclosing manuscripts. Disclosing those disclosures is a natural follow-up. One way to do it is to simply plunk down a list on a weblog, such as http://amirmideast.blogspot.com/2010/12/alphabetical-list-of-open-access_10.html
A more visual approach is taken by Digitized Medieval Manuscripts. It is in an interesting attempt, to say the least, though far from perfect. It is difficult to assess how complete it is, and it stands to reason that the collection is skewed towards a particular kind of manuscripts, but this is not made clear. It could definitely benefit from topic/thematic sub-maps.
The biggest danger to such lists is if they are not checked and updated; information could be outdated or links could turn dead. Nonetheless, such efforts to create a system out of the chaos of different digitization projects is commendable.