By far, the coolest thing in Mac's OS X 10.5 (Leopard) is the integration between Mail.app and iCal.app, the natural language understanding of dates, times, and locations that merges the two.
When I get an email with a date in it, I can mouse over the the event's date information (even if it's the word "tomorrow"), click "add to iCal" and a little window will pop up with as much info taken from the email as it can possibly extract. Usually, I can then save the event with little to no clean-up.
In the screenshot above shows the topic, location, date, and time-frame extracted. It's pretty wonderful that I can do this and has made me rely on my calendar more than ever.