The current version of the Stack Exchange API, (which covers Stack Overflow) can be found at https://api.stackexchange.com/.

 - [Blog Post](http://blog.stackoverflow.com/2012/09/stack-exchange-api-v2-1/)
 - [Documentation](https://api.stackexchange.com/docs)
 - [App registration link](https://stackapps.com/apps/oauth/register) (required for unrestricted usage)