I started writing some posts (and published one) here about x509 extensions, but I decided to instead create a Wiki for this stuff.
Yeah, I could just contribute to Wikipedia, but PKI is such a niche topic, and so vast, that I feel (for now) that it should be contained in its own hub of information and community to edit that information.
So, check out pkiwiki.com, and start contributing!
Note: This wiki will hopefully extend into other areas related to PKI, like Smart Cards. The world is seriously lacking in publicly available knowledge on these topics.