pfsense is a customized FreeBSD+pf distribution designed for use as a firewall. It wraps many of the features of the pf firewall code in an easy-to-use web interface.
pfSense is an open-source firewall product built on top of FreeBSD and the OpenBSD pf firewall.
It provides a graphical (web-based) interface for configuring and managing firewall rules, as well as viewing traffic and firewall decisions (accept/reject).
pfsense is similar in concept to m0n0wall, however as of this writing m0n0wall uses the ipfilter packet filter.