Tidbits has a nice article on the Hidden Refinements in Snow Leopard. Of all of these probably the biggest one for me is the time for screensaver lock. It lets you be asked for a password when the screensaver comes off, but NOT until after X amount of time. So you can set it so that you don't get prompted for the password if the screensaver comes on for a minute while you're having dinner, but it will after the screensaver has been on overnight.