Thanks to Rachel McAlpine for a link to the National Cancer Institute's Research-Based Web Design & Usability Guidelines, a PDF document that looks very usable indeed.
I've printed several chapters, and they're excellent—a blend of text, graphics and citations. The advice is timely. I'm creating content for an academic Website, and the guidelines are going to save me some grief. They aren't radically different from much earlier rules like Jakob Nielsen's, but NCI expresses them clearly.
Rule 9 on "Writing Web Content" is especially good: "A sentence should not contain more than twenty words. A paragraph should not contain more than six sentences." That's the kind of rule that's easy to break, and we should—but only after we've thought carefully about it.