I tend to be picky about the software I use on my computer - that extends to my choice of color scheme as well. I didn't especially like using any of the popular color palettes for extended amounts of time, and any of the more obscure ones tended to have limited support for the applications I did need support for. So I did something about it.
Taking inspiration from existing color schemes and design cues, I created the Mountain color scheme and theme library - in fact, this website uses that same color scheme. It takes advantage of low-contrast colors and off-black/off-white colors to minimize eyestrain, while retaining a diverse and visually appealing color palette. While much work went into working on and perfecting the colors and themes, I'm more proud of the community reaction to this project. My recent commitments have been dwarved by pull requests from others who appreciated the work I put thus far, and at the time of writing the repository has crossed 200 stars on github. As far as side projects go, this is definitely the one that I feel has had the most effect on other developers (even if not in the most technical sense).