1990s: Solaris
1999: RedHat
1999: FreeBSD
2003: RHEL
2003: Fedora
2004: CentOS
2005: OpenBSD
2012: OpenWRT
201X: Debian (precise date n/a)
Still using most of those in varying system roles, with the exception of Solaris supplanted by FreeBSD (soon to be NetBSD due to Sparc support).