n0tice is a simple location based public notice board system. It's completely open and unfiltered, and allows users to post any sort of notice they want, tagged with their location, and lets them read notices near them, sorted by distance. Quite a simple little idea, and if it gets a user base, it could end up being quite useful. Although first designed for iPhone, the n0tice API was quickly released to allow integration into other programs, including desktop apps.
There isn't just one social graph, there are many. LinkedIn maintains your professional contacts, while Facebook has friends and family (and yes, often many professional). Twitter has some mix, as well, perhaps with a different filter. Aggregators take these disparate contacts and bring them into one place. A new service uses several APIs to merge them and give tools to better understand and utilize your network of contacts.
For a long time the comment section on any webpage was nothing more than internet real estate about to be foreclosed on. Plagued by spammers, pointless shout outs, and unreal amounts of bickering, most users chose to forgo perusing what was once a land of potential user interaction. Livefyre, a designer of Dynamic comment platforms, is working hard at ushering commenting into the promised land, with the Livefyre API to help get us there.