Jakob Nielsen's 10 usability heuristics are nicely illustrated in this article pulling together lots of examples of good practice.
Jakob Nielsen and Ralf Molich first drafted a set of usability heuristics over 20 years ago. Nielsen has since gone on to refine these based on analysis of numerous usability studies through the 1990s and published in his book - Usability Engineering.
His ten general principles for user interface design are called "heuristics" because they are more in the nature of rules of thumb than specific usability guidelines.
They're useful in themselves, but I found this article in which the author picks on elements of popular applications to illustrate good practice to be really useful.
6 Tips for a Great Flex UX Part 5: Review Usability Best Practices - article by Theresa Neil
And if you're new to usability heuristics:
Jakob Nielsen's 10 usability heuristics
Nielsen on heuristic evaluation
Wikipedia article on heuristic evaluation
And a fairly negative take on heuristic evaluation:
6 things you didn't know about Heuristic Evaluations - article by Jeff Sauro