When I was a CS major in 1999-2003, the big names I remember people using were RedHat, SUSE, Debian, and Mandrake was gaining market share for desktop use. The installfests I went to were all RedHat. There seem to be a lot of complaints here about RedHat after that era, but my memory is that RedHat was the main linux distro circa 2003.
Mandrake! What a blast from the past. I also remember Gentoo and Slackware from around the same time, though apparently the latter predates the former by a decade or so.
Slackware in the 90s was amazing. Super stable. I think
I continued to run Slackware until around 2002 to 2004 (can’t say for sure but it was to run an early(ish) version of Ableton for laptop DJing - as I was creating a concept set that just wouldn’t have been possible with vinyl alone), and wanted a distro that was a little lower maintenance given the advances to Linux at that point.
I’ll always have a soft spot for Slackware even if I’d never dream of running it any longer.