Gaming on the fly. Like any GM out there with an ounce of imagination, I had a folder with hundreds of ideas for games that did not fit into the genre of the standard RPG. So one day when a bunch of friend and I had gotten together to watch a movie, afterwards we all went out to a coffee shop and played our first D6 adventure. Basically, I was running a game completely off the top of my head, and they played themselves as the characters in the game.
Each game started the same way. My friends would show up to game and they'd sudden be enveloped in a brilliant flash of light and they'd end up some place else. All they had to work with was what they had with them on their person. (Many a time they'd have taken off their shoes, and would find themselves in a very unforgiving, foot-harming environment.)
This game style allowed me to bounce game ideas off my friends and get feedback on those ideas. It also forced the players to truly use their imagination and their real skill to solve strange puzzles and get out of unearthly situations.