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.
A excellent example of an alternative operating system, but not for the faint-hearted.