Gregory's Server
When I set out to write a reverse geocoder, I had a very narrow scope in mind: basically the east coast of the US. But the longer you study something, the more edge cases you find. Got a few talks out of it:
https://www.youtube.com/watch?v=sjE6xy0BBgc
Juno Reverse Geocoder is covered in tests no other geocoder would pass, and is built on SQL. I bragged that it was faster than Nominatim, but recently I've tested it and it's like 8 times slower. Still, worked fine for our company.