On http://stackapps.com several Mac OS X applications are listed. StackApps is the StackExchange API and application and scripts site.
Personally, I use Newt, which can display new questions, comment notifications and reputation changes from across all the StackExchange sites, including AskDifferent.
To answer some points raised elsewhere:
- The SOStatus app most likely uses the StackExchange API.
- I don't think it is Open Source; the developer does not list it on it's Open Source software page nor is there any trace of it on their GitHub account.
