Google's Google+ History API was recently revealed. It's been compared to Facebook's Open Graph, but uses Google+ connections to bring in updates from third-party services (Twitter, Foursquare, etc.). It also lets you choose what data from these services you want shared. The API collects what users have done with the apps, and adds it to their personal pages for sharing at the user's discretion.
Google has uploaded a new developer Hangout about the API, so you can learn more about developing with it. Specifically, the video looks at a ToDo list application developed using the API and GO on AppEngine.
In case you missed it, here's a session from Google I/O all about the API: