I just came across freebase again, and I have to say this thing looks really prospering. Freebase is sort of a metadata / semantic web wiki, structured around topics, types and domains. Essentially, it lets users add descriptions of entities, such as movies, persons, buildings and relate them to each other. The set of properties used, of course, depends on the type of an entity. The project reuses a lot of Wikipedia or other free information, but the interesting thing is the structured approach and, for developers, especially the really powerful API with a very interesting query language approach based on JSON. Mashup time! :)

To get started, browse freebase, e.g. about, say, architecture!