There isn't much to say about this one. It started out as a desire to play Yukon in a terminal – I could only find a few implementations of Klondike. So I created yuk, a simple ncurses-based Yukon implementation in C. I then realized that most of the code could easily be reused to implement other games, such as Klondike and Freecell. The result was csol.
At the moment, csol includes 5 different solitaire games: Yukon, Klondike, Freecell, Eight Off (very similar to Frecell), and Russian Solitaire (very similar to Yukon). They are implemented in a simple declarative language I designed for csol. It should be possible to implement a lot more games using it, but unfortunately I don't know that many different solitaire games. csol also has support for different themes using the same declarative language.