What websites provide downloadable archives (similar to SE's data dump) of user-generated content under liberal licenses (e.g. creative commons)?
Planet.OSM has geo-spatial data.
Freebase: [...] distributed, like Freebase itself, under the Creative Commons Attribution (CC-BY) license and use is subject to the Freebase Terms of Service. If you include the data from these data dumps in a website or application, you must attribute us as described in the Freebase Licensing Policy.