I use http://wikimapia.org/ when traveling to find out about what's around me. Going around the US by train last year I passed a decommissioned ICBM launch site and a couple of Superfund/nuclear waste sites. Neat.
Shameless self-plug; I recently launched an app to help people discover and contribute interesting things about the places they go - https://www.r3d.city. Feedback greatly appreciated!