Conway's Game of Life^[1] elements are pixels that spawn or unspawn respondingly to the number of cells around it. Also known as a cellular automaton^[2]

Some of the rules used are these:

1. Any live cell with fewer than X live neighbours dies, as if caused by under-population.
2. Any live cell with Z or Y live neighbours lives on to the next generation.
3. Any live cell with more than A live neighbours dies, as if by overcrowding.
4. Any dead cell with exactly Q live neighbours becomes a live cell, as if by reproduction.

These rules are used in all of the Powder Toy's Game of Life variations. For example. GOL has a string saying "B3/S23" which means "These cells are born if they have three cells surrounding them and survive when they have 2 or 3 live neighbors."
REPL has a "B1357/S1357" which means these cells are born when they have 1/3/5/7 neighbours and survive with 1/3/5/7 neighbours.
With these simple elements you can create the most awesome contraptions.

Also, in The Powder Toy, these newer particles(that just appeared) heat up to create a nice show in Heat view. This makes small heater elements like these:

very hot, while these still elements

stay nice and cold.

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^[1]http://en.wikipedia.org/wiki/Conway%27s_Game_of_Life
^[2]http://en.wikipedia.org/wiki/Cellular_automaton