A collection of guidelines compiled by the US HHS for web usability.
The Department of Health and Human Services (HHS) has compiled a number of studies and research data into this document. The result is a number of useful guidelines to consider when writing for the web. Each guideline is rated as to importance and strength of evidence, so you - the reader - can draw your own conclusion as to how important any particular guideline may be. Guidelines include suggestions on presentation (amount of text, color choices, etc), appropriate sizes, designing for the most popular browsers while still allowing for less-popular user-agents (like screen-readers or mobile web browsers), and even minimizing load times and other "hardware" concerns.
While a little dry, this does represent a good (and well-indexed) collection of usability guidelines.