Golly

Golly is an open source, cross-platform simulation of Conway's Game of Life. The game board is a grid of square cells, and each cell lives or dies according to how many neighbors it has: a dead cell comes to life if it has exactly three live neighbors; a live cell stays alive if it has two or three live neighbors, and dies otherwise. After creating a starting position, the player can watch the cells to see what they do, or interfere while the simulation is running to add or remove live cells.

Unlike many other simulations of the Game of Life, Golly features an unlimited-sized board, a built-in collection of 'patterns' of pre-arranged cells, and the ability to undo and redo steps. Golly also supports other rules than Conway's Game of Life, including sixteen built-in alternate sets of rules, and the ability to enter others.