Since the Magic Mouse's surface is one big touch-sensitive area, it was only a matter of time before third-party developers came up with solutions for adding custom gestures. By installing free and inexpensive system extensions, you can assign commands to various swipes and taps.
Since your Mac can handle more than one audio device, it is important to know how to tell it which device to use for output and input. For instance, you can have external speakers and a USB headset, as well as several microphones. You can set this system-wide, or for individual applications like iChat, GarageBand, Audacity and Skype.
It is easy to hook up a second monitor to most Macs. Once you get a monitor and the proper cables to connect it, you can use the Displays preferences to control how the monitor fits into your desktop.
A basic look at Energy Saver Preferences. These can be used to reduce the power consumption of your Mac when you are not using it. You can also schedule shut down or sleep patterns, and set how how Mac behaves while it is asleep.
With the Ink feature of Mac OS X you can use a cheap tablet and your Mac will recognize the letters you write as an alternative to using a keyboard. You can insert text and drawings into documents.
Spaces creates virtual displays that you can switch between even though you only have one physical screen. You can place different windows and applications in different Spaces to extend the amount of desktop space you have to work.
You can change your system alert sound, even using a custom one that you create.
Learn about Hot Corners and how you can use them to have your cursor trigger actions like starting or disabling your screen saver, putting your display to sleep, or starting Expose or the Dashboard.
Learn how to use Expose to make it easier to navigate between your windows and applications in Mac OS X.
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.