[rwmj]

The Wikipedia page links to some papers (if you can find them - they are online but paywalled sadly).

In practical terms I found Tierra and my system to be very interesting to observe.

(Since I can't edit the original comment above ...)

Here's the description I wrote of my artificial life system: http://oirase.annexia.org/dlife/machineref.html

And here's the "god cell", the initial hand-written cell, from which all other cells evolve: http://oirase.annexia.org/dlife/god.dla