UPDATE (2007.07.02): It seems that the IP-restricted URL that used to be here doesn’t provide access; you’ll have to go to the (password-protected) staff database trials page to get the username and password that does work.
From the vendor:
ArchiveGrid is an important destination for searching through historical documents, personal papers, and family histories held in archives around the world.
Thousands of libraries, museums, and archives have contributed nearly a million collection descriptions to ArchiveGrid. Researchers searching ArchiveGrid can learn about the many items in each of these collections, contact archives to arrange a visit to examine materials, and order copies.