Geographic data, especially postal codes. In a lot (most?) countries the postal office is sponsored using tax payer money, but acts like a corporation, sells the data and adds various restrictions to the data. Newest example is Ireland where postcodes are about to get introduced: you can't get a full list of postcodes without paying.
That's just diaspora, or any of those open source apps that let you post/view feeds. Nothing interesting. Open source the graph search algorithm and perhaps..