The first real work I did was with OpenBSD - firewall/gateway/proxy boxes, mail relays, etc. I poked around with Linux before, but not really for serious stuff. Later I got into Linux for work stuff because I had a buddy that knew more about Linux than I knew about BSD.
I've worked with some SCO Unix at one point, and these days I work 95% Linux and 5% FreeBSD and Solaris. For my daily work I log into about 20-30 servers per day (from a pool of around 60,000 last time I checked). Roughly once a week I get a box that has Debian, Ubuntu, SLES, Gentoo (shudder), or Red Hat 6.x-9 on the really old boxes. The rest of the time it's RHEL or the odd idiot who insists on running Fedora or CentOS despite paying for an RHEL license.