Homepage: http://www.openbsd.org/
Latest version: 3.4
OpenBSD is a pro-actively secure Operating System maintained by a team of volunteers. The team systematically reviews their source code to eliminate bugs and vulnerabilities. When a flaw in another OS is made public, the OpenBSD team checks to see if similar problems exist in OpenBSD. The OpenBSD team is the same group of people that make the widely used OpenSSH package.
The OS is well suited for building firewalls, secure web servers, and hosting shell accounts. The OS is lean, with the default install requiring about 200MB of disk space. It includes a ports collection (similar to FreeBSD's) that allows the easy install of thousands of open source packages.