Links:

The indispensible FreeBSD Handbook

The great OpenBSD Frequently Asked Questions

For NetBSD there is The NetBSD operating system, a short guide

A new guide - recommended! Comprehensive guide to FreeBSD

Informative FreeBSD site: FreeBSD Diary

The very comprehensive site on UNIX and BSD in general: VMUNIX.com

Also try; www.netcraft.com.

The FreeBSD Browser : www.cfcl.com/Meta/md_fb.html

The FreeBSD Forum site: www.freebsdforums.org

Greg Lehey's The Complete FreeBSD: [PS] [PDF]

Operating system book with a chapter on FreeBSD: Read the chapter here

Slashdot BSD section: SlashDot

Google BSD search: Google