About a year ago I got a Tesla K80 off eBay for about $200. It's basically 2 Tesla K20Xes in one card, so it's pretty powerful, but the downside is that it's a datacenter GPU - it doesn't have any cooling.
I 3D printed a fan shroud for it and put an old (but surprisingly powerful) fan on there, and that worked ok. The issue, though, was that I wanted the fan to be quiet when the GPU was idle, and I couldn't figure out fan control on Linux, so I decided to control the fan speed via an Arduino.
I took a spare Arduino, some wiring, and despite never before programming anything for Arduino, I managed to hack this together: https://github.com/askiiart/k80-linux-cooling
It takes the GPU temperature, turns that into desired fan speed, then sends the desired speed to the Arduino over USB. The Arduino just controls the fan speed via PWM.