A controller for a strip of NeoPixels, which I currently have attached to my desk at home. The desk has a frosted glass surface, and the LED strip runs around the perimeter of the desk underneath, making a nicely customizable light display for my work area.
Aside from being a basic colour picker, it also has a marquee mode, a pulse mode, and my personal favourite, a gently shifting rainbow mode. I find myself putting this on a lot when I'm sitting down to work; somehow the stimulation of pulsing colours helps me to focus, kind of like visual background music.
It's controlled from a web panel written in React.js, because I wanted to try React out. It's also got a couple of quick command line shortcuts. The server connects to an Arduino to send data to the LEDs, and was originally written in Python, but I later switched it to Node.js since I'm doing so much of that lately.