The Atlantis library will aid you in mocking the Internet for your Android app. It offers means of delivering custom tailored responses to network requests made by your app, without forcing you to do massive changes to your code.
The "News" section in the Wiki holds a rough change list. You can also get a quick-n-dirty introduction to Atlantis in the "How to" section, or have a look at the JSON configuration reference and the JavaDoc documentation for more technically detailed information in the "Reference" section.
Feel free to raise a ticket if you find a bug, would like something to change (actually I'd consider buying you a beer if you submit a pull request with a suggested change), or just want to discuss a topic.