Openbsd PF : The OpenBSD Packet Filter
Like Netfilter and IP Filter on other platforms, OpenBSD users love PF, their firewall tool. It handles network address translation, normalizing TCP/IP traffic, providing bandwidth control, and packet prioritization. It also offers some eccentric features, such as passive OS detection. Coming from the same guys who created OpenBSD, you can trust that it has been well audited and coded to avoid the sort of security holes we have seen in other packet filters.