This application is the original version of the upgraded GTK+3 project checkers3 featuring GTK+2
Because it's fun and implementing it provides a good insight into a language/toolkit.
See Automake
./autogen.sh
make
make install
(if you're enthusiastic enough)
The app was written on Linux but it supports other platforms as well like Windows or OSX. See Other OS Support for details.
This project comparing with its successor checkers3 can provide some hints about what needs to be changed in the code in order to make it work with GTK+3. Of course it's not a very sophisticated project so many gotchas necessarily left out, but the basics seem to be covered.
This version was written about ten years ago and never really finished (although working as a game). It lacks the features intended for Gnome3 "integration".