Video game speedrunning - playing a video game over and over, trying to beat it in the fastest time possible. It scratches an engineering itch to know miniscule little details and optimizations about a game that you love, and be collaborating/competing with others, working to push down times. It has a chill social aspect to it, where you stream on twitch and your friends watch your attempts, talk about the techniques, and hang out with you while you practice. A lot of incredibly bright, talented and kind geeks. Some of the real scientists behind the scenes are brilliant engineers who poke at ram values and theorycraft routes based on manipulated RNG seeds.
I got into the scene from participating in a speedgaming tournament (http://speedgaming.org/events/), which forced me to learn a game and made me several great friends.
Since then I've gotten deep into a zelda randomizer (http://alttpr.com/) that has a vibrant and passionate community. A really smart engineer realized that you could randomize the items in the game by breaking it down into a dependency graph, so that the game is different every time. You race on the same seed as someone else, and you win by efficiently routing your item checks, better faster at the game, and/or making good gambles. It's one part speedrunning, one part poker.