Tierra[0], written by Tom Ray[1], immediately comes to mind. I was captivated when I read about it, as a teenager, in Steven Levy's "Artificial Life"[2]. Having played Core War[3], the description of Tierra in Levy's book inspired me to play around with making a virtual machine in Turbo Pascal and trying my hand at making a pale and naive clone. It was a lot of fun, and arguably has influenced a lot of my thinking about the origin of biological life.

[0] https://tomray.me/tierra/whatis.html

[1] https://en.wikipedia.org/wiki/Thomas_S._Ray

[2] https://www.stevenlevy.com/artificial-life

[3] https://en.wikipedia.org/wiki/Core_War