Data Detector in your pointer
This is pretty great. In an email you receive, your mouse pointer turns into a data detector. If you mouse over an address, a box automatically appears around the address and displays a tiny arrow at the end. Click on the arrow and you get a contextual menu with options, shown below, that includes an instant link to a Google map to that address. It's kind of freaky, actually.
If the pointer detects a date, even such a date as "tomorrow" or "Thursday," you get options to create a new iCal event (in which case you'll get a small window for editing the information and choosing the calendar) or to open iCal and show this date on the screen.
And there are many more small, but significant, additions to Leopard that you should be sure to check out, such as the new Grammar Check and true typographers quotes in TextEdit (finally!). Or the new Guest Account feature where anyone can log in to your computer as a guest, and when they log out, everything they did completely disappears. There are new Desktop images; the iChat Buddy List is now in groups so the list isn't so long; and lots of other things that will surprise and delight you.