Messed around a bit with RPG Maker on PlayStation for a while, but didn't do much outside of designing some levels. I've also Dungeon Mastered a bit in 3rd and 4th edition Dungeons & Dragons.
My favorite experience was probably with Graal, back when it came with a server-running utility and you could run your own servers. Damn that was so much fun. A lot of my friends and I made a ton of awesome towns and dungeons and even learned to mess with events and such a little bit. What a fantastic thing back in the day.