pfSense is a free, open source customized distribution of FreeBSD tailored for use as a firewall and router. In addition to being a powerful, flexible firewalling and routing platform, it includes a long list of related features and a package system allowing further expandability without adding bloat and potential security vulnerabilities to the base distribution. pfSense is a popular project with more than 1 million downloads since its inception, and proven in countless installations ranging from small home networks protecting a single computer to large corporations, universities and other organizations protecting thousands of network devices.
This project started in 2004 as a fork of the m0n0wall project, but focused towards full PC installations rather than the embedded hardware focus of m0n0wall. They also offer an embedded image for Compact Flash based installations now. The two projects have since diverged significantly.