Golly

Golly is an open source, cross-platform application for exploring Conway's Game of Life and many other types of cellular automata.

Key features:
Supports bounded and unbounded universes, with cells of up to 256 states.
Supports multiple algorithms, including Bill Gosper's super fast hashlife algorithm.
Many different types of CA are included: John von Neumann's 29-state CA, Wolfram's 1D rules, WireWorld, Generations, Langton's Loops, Larger than Life, etc.
Reads RLE, macrocell, Life 1.05/1.06, dblife, and MCell files.
Can extract patterns and rules from zip files.
Can download patterns and rules from online archives.
Includes a state-of-the-art pattern collection.
Unlimited undo/redo.
Extensive help, with an integrated Life Lexicon.