Geonames (geonames.org/) provides a lot of data for geographic places. This includes coordinates and markings on a map, as well as other contextual information.
GN250 contains data from the German Federal Agency for Cartography and Geodesy. The entries are contextualized by a type, for example "Landschaft" (landscape).
The GND is the integrated authority file of the german Nationalbibliothek. It provides standardized data records concerning persons, corporate bodies, congresses, geographic entities, topics and works.
Wikipedia provides an OpenSearch Suggest API as well.
RVK via RVK api
GND subjects via lobid.org
GND people via lobid.org