http://www.openbsd.org/cgi-bin/man.cgi - OpenBSD manpage collection is great if you want to learn about the Internals of the OpenBSD system. As when it comes to functions they try to explain how they work and not just how they can be used. This makes OpenBSD by far the best system to:
* Get experience coding in C.
* Learn the real underpinnings of Unix.
* Understand what really sucks about Linux, Windows, FreeBSD, everything.