Cover was a group project that I worked on with two other developers from Upcase as a learning project.
The spec was relatively straight-forward, but it was a chance to pair program with some new people and I learned quite a bit.
Here is one PR that I worked on.
For me, the big value was in seeing how other developers approach TDD. Since I've spent a lot of time as the sole developer on projects, I tend to go overboard with my test coverage sometime. One of the people I paired with was very sensitive to test duplication and only writing tests if there is a reasonable chance that the feature could be broken.
Here's an example where we added a delegate, but did not test it explicitly since it was implicitly tested in the feature spec by the implementation in the view. I think I still prefer the explicit unit test, but it's an interesting approach.