My initial exploration into game modding was with Rise of the Triad and Duke Nukem 3D. Releasing these works online around 1995/1996 led to my first professional industry projects with Legend and Sunstorm.
While at 3D Realms I got the chance to put together the Duke Nukem 3D and Shadow Warrior source code releases. Since these games played such a big part of my industry start, being able to make them available as open source for the broader community was a dream come true. Working on them was a bit like the story to Jurassic Park, where I had to plunder similar but not original code from various places in order to piece them back into working order, as well as outright writing some new bits to fill in the remaining blanks.