OpenBSD was a split off of NetBSD. Theo de Raadt had a conflict with the other core members of NetBSD, and was cut off from the development tree. He then started work on OpenBSD by auditing all of the NetBSD code for security holes, and places where the code was written in a non-standard way. As such, OpenBSD is very secure, and stable (due to its cleaned source). OpenBSD also is nice because the OpenBSD team has rewritten most of their man files, as such, they tend to be clear, concise, and make sense. NetBSD runs on many platforms, and is a pretty good OS overall. FreeBSD runs very fast on x86 hardware, and it comes with a very good ports tree, but it is far more insecure then OpenBSD.