These are my most used Linux/Unix server distributions:

Fedora Core release 6 (Zod)
Fedora Core release 4 (Stentz)
CentOS release 4
Debian GNU/Linux 4
Red Hat Enterprise Linux ES release 4 (Nahant)
Debian GNU/Linux 3
FreeBSD 6
CentOS release 5

The way to get your linux version

cat /etc/*release
cat /etc/issue
uname -a
nmap -sS -O -A host