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.
Some Twitter users may have, at some point, had the need to share a conversation or a string of tweets with other people, and would have been left frustrated with the discovery that there wasn't really a way to do that. This was the very frustration that fuelled the creation of TweetVue; a tool that makes it possible to share Twitter conversations with other people. In addition, TweetVue's API gives developers access to this data, allowing them to access and share these conversations.
Keek, social video network, recently announced the release of the Keek API. Alongside the API launch, Keek established a developer portal that includes tools and resources to assist developers in integrating Keek with third party applications. The Keek API grants developers access to public keeks (i.e. 36 second video updates), search, user profiles, klusters, and more.