Triumph of the Mod
Salon.com has a cool article online today called "Triumph of the mod". In it, the article talks about how customizing computer games started. They talk with Tom Hall of Monkeystone (formerly of id, Ion Storm, & Apogee), as well as Scott Miller of Apogee/3D Realms and several others. They talk about some of the earliest known total conversions (such as the Smurf conversion for the Apple II Castle Wolfenstein game), up to Doom mods, and things such as Valve's Steam. Here's a bit of what Scott had to say about the early days of user modifications:
Scott Miller, now CEO of 3D Realms, first noticed that enthusiasts were creating levels for the original "Duke Nukem" (1990), developed when Miller was at Apogee. Not only were gamers creating mods for his company's games, they were even creating level editors to simplify the process for making them -- then distributing both to other gamers online. "This was a fascinating development," says Miller. "We just didn't expect players to take the time and effort to create their own development tools."
Check this article out, it's quite good stuff.