OpenBSD is a Unix-like operating system, a descendent of BSD, a Unix derivative developed at Berkeley University in California. OpenBSD started as a fork of NetBSD and its focus is on open source code, security and excellent documentation. Software licensing is a key concern, so don't expect to find any proprietary software in here. While it is traditionally viewed as a server platform, due in no small part to its very strong focus on security, OpenBSD does have a good choice of desktop software, although not as in-depth as say, Linux. It uses X for the graphical display and has a choice of desktop environments, including GNOME, KDE and XFCE.
The size of the standard download (232MB) shows that this only installs a minimum system, you then need to pick and choose the software you need for whatever you intend to use the computer for.
OpenBSD runs on seventeen different hardware platforms (at the last count) from the standard x86 and 64 bit Intel-based hardware, through various Apple hardware to devices as far from the mainstream as the Sharp Zaurus and Amiga. You may even be using OpenBSD to read this without realising it, as it is used in several commercial networking products such as routers and firewalls.
A excellent example of an alternative operating system, but not for the faint-hearted.