Jecology (James' Game of Ecology) is a life simulator created in the spirit of Conway's Game of Life.  I have always found Conway's Game of Life(CGoL) interesting in an academic way, but somewhat limited in the scope of simulation.  There is only one type of creature in CGoL.  They reproduce, but do not mutate or evolve.  They do not have to find food, but instead simply die based on scarcity or overpopulation.  Jecology addresses these shortcomings with a more complex simulation,  but retains much of the elegant simplicity found in CGoL.

More screenshots with explanations of what's happening.

I like to think of Jecology as a full ecology simulator. There are inanimate lifeforms (these can be thought of as plants, or plankton) in Jecology that serve only as stationary and renewable food for the lower animate creatures. The "world" or "universe" starts with only the lowest form of animate creature, which can reproduce asexually, and also occasionally spawns mutations that can be construed as either harmful or beneficial.

Animate creatures are very simple, and merely head towards food.  However, the rules of the universe cause them to form complex "societies" or hierarchies.

Here's some interesting command line options you can try:
jecology -t:0.1   
    This runs really fast.
jecology -i:5
    Animate lifeforms starve very quickly at first.
jecology -b:10
    Scarce food at the bottom of the foodchain.
"jecology -h" will list all the supported command line options and "jecology -d" will show you the default settings.