The Knowledge Graph Search API lets you pretty much query data within Google’s Knowledge Graph, but doesn’t let you push changed to the Knowledge Graph. You can suggest edits the old fashion way within the core search results.
The API can be found at developers.google.com/knowledge-graph. It lets you, for example get a ranked list of the most notable entities that match certain criteria, predictively completing entities in a search box and annotating/organizing content using the Knowledge Graph entities.
Google also said they are working on a “replacement for the Freebase Suggest Widget,” and when that is done, Google will discontinue the Freebase API three months later.
Here are some of the entities within the new Google Knowledge Graph Search API; Book, BookSeries, EducationalOrganization, Event, GovernmentOrganization, LocalBusiness, Movie, MovieSeries, MusicAlbum, MusicGroup, MusicRecording, Organization, Periodical, Person, Place, SportsTeam, TVEpisode, TVSeries, VideoGame, VideoGameSeries, and WebSite.
Forum discussion at Google+.