If youre willing to lose yourself for a few hours and explore some seldom seen historical documents then check out the National Archives Digital Vaults. The site employs a tag-based interface to connect photos, letters, government documents and cultural artifacts all by topic.
Examining a photo of the infamous Zapruder camera linked me to photographs of Lincoln assassination conspirators, a Jello box used as evidence in the 1951 Rosenberg spy trial, an image of the Kennedy Space Center taken from an orbiting space shuttle, and a photo of Deng Xiaopings visit with President Carter in 1979.
Digital Vaults is a good example of non-traditional navigation and a 100% flash-based site that works because of its nature. If I wanted to do serious research using material from the National Archives, I would expect a site organized more like Wikipedia. But Digital Vaults is like a museum that allows you to wander from gallery to gallery and make unexpected discoveries.