Very clear and easy to use http://www.whitehouse.gov/ Go past front page for clear example of nav menu
and I think for a business application one could do a lot worse than copying The Economist Online http://www.economist.com/?sa_campaign=econsubs/asia/subspage/textlink
The Economist on iPad and their sister (free for now) iPad app Intelligent Life are spellbindingly well done. (and I only recently came across some chatter that agreed – this really is my own opinion)
Some good checklist stuff here http://www.maxdesign.com.au/articles/checklist/
for CSS links, eg: Guidelines for visualising links http://www.useit.com/alertbox/20040510.html