Amazingly, security libraries are often in this category. Is there a really good writeup ANYWHERE about SSL, certificates and signing practices? And IPSec with all its intricacies?
Funnily enough, on Stack Overflow! Not all of the security-related questions are overflowing with shitty misinformation. (SO might not be great, but it's better than the squillion shitty places for question answering that preceded it.)