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!