Modding in the playground

[Lvl 2 Expert]

This is a thread for talking about modifications you made for games. Nothing's too small.

Me, I only ever modded one game, my old love Stunts (https://en.wikipedia.org/wiki/Stunts_(video_game)).
research done and tools created by members of the community (http://stunts.hu/) have made it possible to add cars to this old gem, and they've created easier ways to make tracks as well. So a few years ago I made a car and recently I felt the urge to start doing it again.

But that's just peanuts, of course. I know you guys can top that. What did you add to a game? Skins? Characters? Story lines? Game modes? A satisfying ending? A way to play Doom and watch the original Star Wars trilogy in game? Maybe some Minecrafters here who did stuff that technically wasn't modding but did add cool content to their game world? Show me the money.

(Console and phone games and such welcome, I figured the tag "computer" showed the intentions of the thread best.)

[Hunter Noventa]

I remember spending hours making bizarre little mods for Civilization 3. Granted, they mostly consisted of pulling a bunch of custom units off civfanatics.com and mushing them together so I had space marines and giant robots and such, all limited to the single faction that I would play and cackle and I blew past everything.