In the course of going through the literature on open access (OA), I've come across a ton of resources for folks who want to learn about open access. Most of them are okay. A few of them are gems. Here are the gems.
Anyone who writes and publishes (researchers, authors, students, and those who want to work with them) should read this little document on Author's Rights [pdf] by Charles Bailey that gives a clear how-to and why-to introduction on getting started with open access. It covers reasons for making things open access, explains copyright and publisher agreements, shows you how to quickly find (and respond to) the policies of the publishers you use, explains creative commons licences and open access journals... I found it to be an easy-to-read guide (10 pages not counting cover page and bibliography) that I'll use as a quick-reference in the future.
For anyone who wants to give a quick talk on open access, use or remix Isaac Gilman's sharp little (public-domain!) slidedeck on "open access in 15 minutes or less."
Finally, if you're an open access advocate at your institution and looking for some empirical data to take to people, print out this handy crib sheet for conversations on open access with both authors and administrators. It contains a list of selling points for each audience, plus (I love this) links to actual data and examples to back up as many points as possible.
It doesn't quite cover everything, though, so you'll also want to download all 17 pages of The Open Access Citation Advantage by Alma Swan, which is an annotated bibliography covering (as of 2010) the empirical research that's been done on the effects of OA on, quite simply, "the things researchers get evaluated on." It's essentially a big table of papers and their findings, so you can (for instance) search for studies specific to your field, sort by sample size, check their analytical approaches, and so on. They consistently find that, yes, Virginia, there is an OA citation advantage. (Of the 31 studies covered, 27 demonstrated a clear OA advantage, and 4 demonstrated either no advantage of a disadvantage -- if anyone takes a look at those 4, I'd love to hear what you find!)