Google Maps within Mail!
If there was one feature that was heavily advertised by Apple that is new to Leopard, it is that Mail recognizes addresses and lets you add them as contacts to Address Book. One thing that this also does that was much less advertised is that you can also load the address in Google Maps. It is simple, but very useful, you just have to click on the arrow in the lower right corner of the recognized address and choose "Show Map...". It will load Google Maps at the right location in Safari!