Coolest thing is hard... I'm a bit of a nerd, but let's go from a few angles.
As a kid I had made and recreated a number of games on my TI-83+ and did don't fun optimization challenges to get as much in the pure BASIC code as possible. I was working in an ARG into it. But all that code is lost because I didn't know how to back up that stuff back then. (And I was a bit lazy even when I knew I should.)
I'm proud of how fun my Football mod for Binding of Isaac is. It's just an item that give Isaac randomly bouncing projectiles, like how a football kind of sporadically bounces in real life. I meant to release a challenge where you get ipecac and football to start, and all explosion immunities are removed from the pool. With a short goal since I think that's enough chaos.
But probably from a different angle PySpeedup and DriveLink are libraries I designed to improve code as invisibly to the end user as possible because I got tired of taking PhD coders' code and making it actually work because they don't understand swap space or scheduling. (I've worked with brilliant algorithms at times, but had to correct critical misunderstandings of the computer at times.) I haven't touched the libraries in years, but a lot of time and research went into it, and there was a full test suite and documentation. I don't think the idea is fully without merit yet as the multiprocessing in Python is better but still has oddities, and I don't think there's an RAM aware abstraction in the base language yet? I forget what state I left things in. I know the CI I was using doesn't exist (for free users) anymore though.
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