From Jeremy S Cook on Tindie:
What is it?
PCB originally designed to control cooling fans on Raspberry Pi boards, but can be used with other small motors or DC loads. Can also work with Arduino and other such dev boards.
Why did you make it?
Wanted a way to control cooling fans off of a Raspberry Pi. While some fans have PWM inputs, some do not. Notably, this works well with the GPIO fan control option in Raspberry Pi OS.
Not a full motor driver (i.e. it only drives in one direction) but can be used with other simple DC motors as well. Also, can be used with Arduino et al.
What makes it special?
It’s very, very small, even compared to the first version. In fact, and should therefore be able to fit inside nearly any case. The 90º headers are even spec’d out to be low profile.