Have you encountered Conway's Game of Life? If you haven't yet and own a PSP, then you may want to grab and install Metroid Life, the PSP adaptation of Game of Life whipped up by homebrew developer dragula96, with a touch of Metroid.

Basically, Conway's Game of Life is a zero-player game where square cells emulate the process of life ans create complex patterns, based on the following rules:

Any live cell with fewer than two live neighbours dies, as if by loneliness.
Any live cell with more than three live neighbours dies, as if by overcrowding.
Any live cell with two or three live neighbours lives, unchanged, to the next generation.
Any dead cell with exactly three live neighbours comes to life.

In Metroid Life, however, Metroids take the place of the simple-looking square cells. PSP users can just leave the program on and watch idly as Metroid cells replicate and die until the entire population dies out either due to overcrowding or becoming too sparse.

Just so you know, the eboot for Metroid Life is not custom, and should work on all custom firmwares. The homebrew was tested on a PSP Slim with 3.90 m33 custom firmware.