For some this is already old news but in case you haven't heard, the Google Page Speed performance testing tool is officially out of Google Labs and in your hands!
Just type in any URL and Google will return a report with an Overview (complete with a percentage score) and high, medium and low priority action items:
Clicking on any item in the menu takes you to more detail:
Remember, these are suggestions, not absolute rules - you may have perfectly good reasons for setting expiration for cacheable objects at 30 minutes or 6 hours instead of one week as Google recommends. But this tool is a great cheat sheet for finding low hanging fruit in optimizing your site speed, which results in reduced bounce rates (and more opportunity for conversion), more returning visits and better SEO!