By default this cellular automaton uses Stephen Wolfram's rule that a filled cell stays filled and an unfilled one becomes filled if it has exactly one filled neighbor.
You can vary the rules and even include indeterministic rules. For instance, one can set chance the probability of generation when there is one neighbor to 0.5, so the generation parameters are 0,0.5,0,0,0,0,0. These more snowflake-like to me than the defaults (see the light blue image). Another interesting set of generation parameters is 0,0.5,0,0.1,0,0,1 (0.5 chance of generation with one neighbor; 0.1 chance with 3 neighbors, certainty with 6 neighbors).
Christmas 2017 update: Added join mode, where neighboring nodes are joined with a fat line instead of there being a hex at them.