The DMMapp (Digitized Medieval Manuscripts App) is a website that links to more than 300 libraries in the world. Each one of these contains medieval manuscripts that can be browsed for free. The DMMapp is a product of Sexy Codicology, an independent project focused on medieval illuminated manuscripts and social media. It maintains a great blog about medieval manuscripts, especially those that are available online.
The beautiful library of the Abbey of St. Gall in Switzerland contains over 2000 manuscripts, including hundreds from the Abbey's glory days of the 9th and 10th centuries. The library is open to the public and to scholars, and the Codices Electronici Sangallenses project is making selected codices - 436 so far - available online.
Visitors to the Morgan Library in New York will have a rare opportunity to view one of the great masterworks of medieval illumination, the Hours of Catherine of Cleves. But if you don't have a chance to visit, all 157 miniatures have been digitized.
The University of South Carolina recently completed an ambitious survey of all medieval texts in the state for an exhibit at the university library. All the works were scanned and archived electronically. However, not only can you view the texts online, you can hear the university's chorus sing (MP3) the musical manuscripts. [more inside]