I looked it up: Xonotic was the answer!

It was actually quite fun, even though I had to compile from source to play on an outdated office server :)

https://xonotic.org/

Oh right! Nice. I've only played it once or twice, but now I'm kinda thinking I should give it another try -- forgot how good it looks. People at Cyan were playing this (and you joined up)?? That's awesome haha