OpenBSD

http://www.openbsd.org/

Only one remote hole in the default install, in more than 8 years!

The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system. Our efforts emphasize portability, standardization, correctness, proactive security and integrated cryptography. OpenBSD supports binary emulation of most programs from SVR4 (Solaris), FreeBSD, LiNux, BSD/OS, SunOS and HP-UX.

OpenBSD is freely available from our FTP sites, and also available in an inexpensive 3-CD set. The current release is OpenBSD 3.5 which started shipping May 1, 2004. The CDs (and Shirts) can be ordered...

OpenBSD is developed by volunteers. The project funds development and releases by selling CDs and T-shirts, as well as receiving donations. Organizations and individuals donate and thus ensure that OpenBSD will continue to exist, and will remain free for everyone to use and reuse as they see fit.

http://www.openbsd.org/

also see: BsD FreeBSD NetBSD