Modding
SMACSid Meier's Alpha Centauri supports a rich variety of mods. The most common are mods that change the rules of the game or the world by modifying alpha.txt or alphax.txt. This is the easiest and one of the most flexible ways to modify the game, and plenty of players use these files to tweak the game to their satisfaction. See Alpha(x).txt editing for more.
Custom factions are another popular modding choice, and an easy way to get started. Here's how: Faction.txt_editing_guide.
It's also possible and easy to edit almost any of the art in the game and to produce your own tilesets, native life, etc. BUncle has written some articles about that, and you can check the forum for more.
The best way to see what's possible is really just to check the Modding subforum :)
The old Game of The Month projects also showcase what you can do with the game.
