A couple of articles that pull together a range of guidelines - from big companies and from governments.
The articles have been pulled together by Justin Mifsud on his Usability Geek blog. A pretty geeky thing to do, no doubt, but a great resource to bookmark.
Of course, in the main it's the companies guidelines that are of most value but there are a few government guidelines worth a look - particularly in the United States.
Companies include amongst others Microsoft, Adobe, Apple and Google.
Official Usability, User Experience & User Interface Guidelines From Companies
Official Usability & Web Site Guidelines Of Governments From Around The World
The UK entry is out of date now, but I am aware of a new initiative - alpha.gov.uk.
Webcredible's usability review of alpha.gov