Two "open collections" are currently accessible: Women Working, 1800-1930, and Immigration to the United States, 1789-1930. They are currently developing two additional collections: Contagion: Historical Views of Contagious Disease and the Islamic Heritage Project. These collections include digitized books, maps, photographs, pamphlets, etc.

Other online collections made available by Harvard University include: Digital Scores from the Collections of the Eda Kuhn Loeb Music Library, Hedda Morrison Photographs of China, and the Latin American Pamphlet Digital Collection, to name just a few.
For those of us without means to travel to Harvard to utilize their rich resources of archival and historical material (primary sources), these digitized collections open up a whole new world. Check out Harvard University Library's Open Collections Program as well as their other web-accessible collections.