The Repustate C# SDK by Repustate is available for download from the official site. It provides API communication to implement sentiment analysis in social media with the aim to improve business reputation.
Crowd.fm is an easy-to-use platform created to promote events online. It's been described by its founder Tekin Suleyman as "an amazing platform for getting events listed with a single click." The company provides the Crowd.fm API to provide developers integration options.
The Facebook Graph API is great and in one year has thoroughly upgraded Facebook's approach to its platform. The Graph API is much simpler than previous iterations of Facebook's APIs and Connection methods. Of course, with the simplicity of the Graph API, it's still easy to get overwhelmed by just how much information you can easily (with explicit and secure user permission, mind you) get access to and use in your apps. Facebook employee Simon Cross put together a handy Facebook Graph API Explorer to help you navigate the Graph API response content.