Over the years I've developed a decent number of add-ons of various types for games, which I really didn't realize until just now. Sure, like many other nerdy children and adults I created Doom levels, Starcraft maps, and Morrowind and Oblivion mods, but I've also done a decent amount of coding for games that were not my own creation.
Working to change or add onto a game in some way is, at first glance, sort of odd. You're working on something that's supposed to be fun and relaxing. But, software development can be very zen and pleasurable in the right circumstances. And, it's nice to have a change of pace and a variety of different challenges. You just have to forget the fact that the amount of enjoyment you'll directly gain from the finished product is quite unlikely to be worth the amount of time you put in—you have to make up the rest by sharing the add-on with others or the inherent joy you get from programming.
Anyway, the mod projects I'm working on for World of Warcraft are certainly my largest bodies of work, as I've probably spent a good hundred hours on them so far, but some of the other stuff I've done has been amusing as well. Sadly, I have almost none of it today; it's lost somewhere in a landfill or on 5.25" disks at my parents' place. As far as the stuff I remember goes...
I wrote a no-CD crack and performance enhancement for the original X-COM. It wasn't a crack in the traditional sense; I didn't have to actually change any of the game's EXE code. I just developed a system of bypassing CD checks, installing additional CD content onto the hard drive, and replacing parts of the game scripts to make things run faster. It ended up being much faster, which made the game quite a bit more enjoyable, as the base game was frustratingly slow. I was about thirteen at the time, and feared getting in trouble if I shared such a thing with the world, so I was the only person to ever experience this speedy version of X-COM. I think my brother and dad both hated the excruciatingly turn-based game, so I doubt even they ever ran my enhanced version.
Another fun DOS-era project was a program I wrote to let you create maps for the Warcraft II expansion. Originally, the map editor only supported maps for the base game, not the expansion, so I added support for the new graphics tilesets. It was just a few hex edits on the map files, but I was pretty proud of it at the time.
One of my first Windows programs was a map editor for a DOS fishing simulator game my brother bought and enjoyed. I don't even remember the name of the game—something like Bass Pro Tour. Anyway, while I didn't find the game fun, I was pretty excited about the idea of creating a whole map editor for the game, complete with some terrain smoothing features and all sorts of fun things. I think this is the only instance in which I spent quite a bit of time creating an add-on for a game I didn't care for at all.
There was also some RPG game I had that only allowed a single savegame, which I found annoying, so I wrote a program to let you keep a whole library of savegames, back them up, and choose the one to play with each time you run the game. It might have been this game, about which I was quite obsessed at one point in my childhood. I probably wrote this when I was about 10.
I know there have been others, but I can't quite recall them right now. They're very pleasant memories, though.