Learn how to use Network Locations to allow your Mac to connect to different networks with different settings as you travel between locations.
Learn how to capture the entire screen or only a portion, then compress it to send it in an email. This is handy for sending bug reports or pointing out problems.
Whether you are having trouble with some of your fonts, or you just want to see which fonts you have installed and organize them in a better way, Font Book is a handy application that comes with Mac OS X Leopard.
Gary Rosenzweig talks about using Leopard's parental controls to keep your kids from playing with things your computer and the Internet that you don't want them to.