pfSense is a free, open source customized distribution of FreeBSD tailored for use as a firewall and router.
OpenSource forks of pfSense:
IPFire is a hardened Linux appliance distribution designed for use as a firewall.
Endian Firewall Community (EFW) is a turn-key Linux based security software product designed for home that can transform any unused hardware appliance into a full-featured Unified Threat Management (UTM) solution. Endian Community is designed to make security simple and help protect home networks by using the power of Open Source.