Remember Counter Strike was not an independent game. It was a mod of Halo. Halo did not offer a downloaded level editor.
Gosh, it's still around, and its web site looks identical to how I remember it looking 25 years ago! https://quark.sourceforge.io/
I thought it was half life
Science and Industry was a fun mod as well. But yeah I also remember CS ad being at another level right away.
Then there's Ricochet.. :')
Someone else mentioned “natural selection” as well
I believe Hammer has actually been out for more than 2 decades now. I certainly recall trying to recreate my school as a de map.
Apparently this sort of thing didn’t go down so well in the USA…