OPNsense is an open-source, FreeBSD-based firewall distro. It provides a modern GUI and numerous plugins. A great plus is its ease of use, in contrast to some other open-source firewalls in the past. Also, OPNsense is a fork of Pfsense (we wrote about Pfsense, so check out that blog post as well - the link is below).
pfSense pfSense is a free network firewall distribution,