Stanford Libraries is home to four original score manuscripts and one letter. All are freely available for close viewing and download.
Explore the rich resources in the Beethoven-Haus Bonn's digital archive. Includes manuscript scores (some with accompanying sound files), letters and other text manuscripts, and a trove of pictures.
See also, the Ira F. Brilliant Center for Beethoven Studies at San Jose State University. There are a number of digital resources on offer including "Beethoven Conversations" and a small image gallery.
Finally, the Music Treasures Consortium, hosted by the Library of Congress, includes digitized manuscripts from a number of institutional repositories.