A free 4.4BSD-based system capable of running on multiple platforms, such as:
DEC Alpha-based machines
DEC MIPS-based machines
Hewlett-Packard HP300/HP400 machines
Intel 80386 and up PC compatibles
Most&MC680x0-based Apple Macintoshs
And others.

And in OpenBSD developer's own word:
Pay attention to security problems and fix them before anyone else does. (Try to be the #1 most secure operating system)