Like the name says, FreeBSD is a free implementation of BSD UNIX, and the one that unleashed the little red daemon upon us. It's still something I go back to regularly, as comfortable as a pair of old slippers.
- miniBSD - FreeBSD in 22MB.
- m0n0wall - an amazing Firewall that boots off CD and stores its configuration on a floppy (supports 802.1Q VLAN tagging, captive portal with RADIUS authentication, the works).
- pfSense - m0n0wall on steroids (more device support and extra features)
- FreeNAS - NAS server that can boot off a USB key.