The Hermon Dunlap Smith Center for the History of Cartography Maps, mappers, and the history of mapping, with slide shows, online exhibitions (e.g. The French Empire in North America, popular cartography), and journal articles. Part of the Newberry Library, Chicago.
The National Maritime Museum, Greenwich has some excellent online collections related to maritime history and technology, including telescopes, marine chronometers, sundials, and a whole lot more. Some stuff I've been looking at: John Harrison's chronometers (described in Dava Sobel's book Longitude), polyhedral sundials, and pocket globes.