Usability.gov Guidelines

In one of my classes this semester, Internet Applications, we've been reading through a publication available from usability.gov:

Usability.gov is the primary government source for information on usability and user-centered design. It provides guidance and tools on how to make Web sites and other communication systems more usable and useful.

I've been really impressed with the information available from this site and the thoroughness of  the guidelines, which can be downloaded chapter by chapter, or as the full publication The Research-Based Web Design & Usability Guidelines (PDF, 20.64MB). Ironically, these guidelines are only available as a PDF—there is no HTML equivalent which is an accessibility failure.

If you're looking for a quick reference for website best practices, I highly recommend you check out the resources from usability.gov.