Saying this because I didn't know it for years: Fourmilab isn't a "they", it's just the personal website of John Walker, the founder of Autodesk and a fantastically interesting person. Not to be confused with "fermilab" of course, which is a United States national lab that does high-energy particle physics.

There are a ton of very interesting things on his site, including a DIY RNG that uses a radioactive source and an event counter: https://www.fourmilab.ch/hotbits/hardware.html

I wonder if anyone else has built a similar device to this, it seems like it'd be very popular among hackers.