Conway's Game of Life in C. Uses a handy library from Notre Dame.

I'm planning on adding a feature to let you step backwards, rather than just forwards. This is an interesting problem because there can be more than one possible backwards step.

I'm imagining a couple possible modes of operation. One in which you get to manually select which possible backwards step to take, another in which it automatically picks one, and a final mode where it'll branch on each step and end up creating a tree of possible moves for a given number of backward steps. That last one probably gets exponential, so maybe I'll try and do some dynamic programming type thing.