I've spent the last 5 months or so working on a custom-built mouse, for the fun of it. Designed the PCB around a high-end laser sensor, designed the case in FreeCAD, wrote the firmware. I've made two so far and given one of them away. I'm planning to make at least a few more. By the time I get 5 or 6 built, I'll have probably spent $1000 on the project, although a majority of that is a 3d printer.

I'm still not sure if I want to try and monetize that or not. People have expressed interest in buying them, but I've actually done the electronics production line thing in a long-ago internship and I know that it really is a job. Also, it's not like I'm going to be retiring off the proceeds of selling weirdly-shaped 3-button wired mice with no scrollwheels, it's a hell of a niche.