Mosaic Maker
This is a bit of a strange one. It started off as a Latin class project in which I was tasked with designing a Roman-style tile floor on a piece of graph paper. Me being me, I wanted to try incorporate some math and computer science into it. I came up with a cellular automata-like pattern, and after a few hours, I had filled the graph paper with a neat pattern of dots. While it looked cool (My sister thought it looked like a QR code, so I tried scanning it but it failed), I was almost certain I had made a mistake in the pattern somewhere. So I decided to automate it. After several hours, a lot of frustration, and maybe a damaged computer battery (my computer gave me a warning about the battery soon after, but it could be completely unrelated), it works. I was going to leave it at that, but my mom suggested that I share it with my class, so I ignored the battery warnings, added some CSS and customization features, and sent it off.
Edit: I ended up taking my computer to the Apple store and they said they'd like to keep it to figure out what went wrong and they'd give me a new computer for the price of just a new battery. So now I've got an M1 Mac. (the old one was Intel)