If you came from elsewhere, here is a canned summary of OpenBSD:
A sanely constructed modern Unix (descended from 4.4BSD), with a well deserved reputation for security emphasis. It is also the home of OpenSSH, PF, OpenBGPD, OpenNTPD, OpenSMTPD, OpenIKED, mandoc and soon LibreSSL.
BSD-licensed of course (with some GNU due to gcc, others through accidents of history), any new code is under the short BSD or ISC license.
A new release N.m+1 every six months (May 1st and November 1st), last two releases supported with patches and a -stable CVS branch.
A lot of work has gone into making a system that is
secure (correct, haz real cryptoz, actively developed and maintained exploit mitigation that's on by default)
usable in real-world contexts
For exploit mitigation techniques, read Theo de Raadt's presentations - (2005 version plus summary of 10 years' work)