I learned HTML thanks to Operation Flashpoint so that I could write mission briefings in the editor...one thing led to another, and I have a successful career as a software developer. Thanks, Bohemia Interactive.
I used to open up mods (I seem to recall them being in "PBO" files) and tweaked around the script code to change things and make my own weapons and such. Also a formative experience on my path to becoming a developer.
I remember the day I bought that game at my local dealer. It looked amazing and nobody told me about it, my friends were playing FFVII and stuff like that. After installing it, it was mindblowing, the mission editor was incredible with infinite possibilities. When you think about it, that engine and it's SDK were really advanced features and concepts. I mean Rainbow Six had a mission planner, but OPF, with it's editor and addons was an inifinite sandbox. Good memories.