These are a few bits of documentation I've written that you might find useful. They are mostly, if not all, OpenBSD-related.
- Beginner's Guide to OpenBSD
- OpenBSD Transparent Proxy Filter
- Introduction to OpenBSD's pf
- Documentation for my libdaemon (libdaemon is a lightweight OpenBSD daemon framework I am working on)
note that libdaemon has moved to tyrfingr.