SquidGuard is a URL redirector used to use blacklists with the proxysoftware Squid. There are two big advantages to squidguard: it is fast and it is free.
pfSense
pfSense is a open source firewall derived from the m0n0wall operating system platform with radically different goals such as using OpenBSD's ported Packet Filter, FreeBSD 6.1 ALTQ (HFSC) for excellent packet queueing and finally an integrated package management system for extending the environment with new features.