Can you hear the collective developer community saying "finally?" After less than three months, Google has made an initial version of a Google Plus API available for developers. The service can access public information on people and the activities the user's have chosen to post publicly, as opposed to a specific circle. In July we asked Is Google Plus Intentionally Late? Google said it wanted to learn how developers will use the service before providing an API. This is likely only the very first version of a Google Plus API.
The announcement post makes sure to point out only public data is available:
Google+ gives users full control over their information, supporting everything from intimate conversations with family to public showcases and debates. This initial API release is focused on public data only — it lets you read information that people have shared publicly on Google+. For example, if you want to get my profile information, you can use the people.get method by sending the following HTTP request
As with many new APIs we see, Google Plus returns only JSON, not XML. Also interesting, OAuth 2.0 is already supported. Currently it appears that authenticating the user only lets you switch out the long Google Plus user ID for a simpler /me, similar to the Facebook Graph API.
Google Plus has several client libraries, likely thanks to the Google APIs Discovery API. The image in the announcement post (see above) suggests that the Google Plus API is in the API explorer, yet it isn't publicly accessible yet.
The Google Plus documentation is hosted on a new Google developer site, developers.google.com. The site previously redirected to a Google Accounts signup.
Previously, there were unofficial Google Plus APIs. That's always a good sign of developer interest, such as when Instagram was reverse engineered. This might be the first time a Google service has had this much interest since developers hacked Google Maps to birth the map mashup months before Google's official Google Maps API was released.