BubbleGrow is a free, open-source, and cross-platform game developed with the intention of being friendly for developers (particularly inexperienced) to contribute to. This is a C++14 project whose only dependency is currently SFML.
To play the game (Windows 64-bit): https://github.com/Salgat/BubbleGrow/raw/master/BubbleGrow-V0.1-Windows-x64.zip
BubbleGrow is heavily documented and uses the Github Wiki as the base for all documentation, including how to build the project and understanding the source code.