Thanks to vadi4 for pointing out Finity Flight: Fight Back:
We are proud to present not only the first commercial game available on MirthKit, but the first commercial game ever to be released with full source code.
Available on Ubuntu, Windows, and soon OSX, Fight Back is the expansion to our August release, Finity Flight, and represents a path yet-untraveled for gaming. It is the first direct hybridization of commerce and the open source philosophy.
Finity Flight: Fight Back is not open source, but it comes close. When you buy Fight Back for $11.99, you automatically receive all of its code and assets. You are free to modify the game as much as you please.
The reason it is not justifiable to say Fight Back is open source is because you are only allowed to be redistribute changes through MirthKit’s arcade. To reiterate, you are always free to publish your changes through MirthKit (you can even sell them), but users will have to gain access to your work by first purchasing the original game.