Vincent Danen shares some of the hidden commands in Mac OS X that you can get to by using the Option key.
Mac OS X has a lot of hidden little secrets that you either need to ferret out yourself, find out from friends who are in the know, or by frequenting sites like hints.macworld.com. A lot of these are hidden preferences that can be changed with the "defaults" command-line tool, but you have to know them in order to find them, so they are generally inaccessible to the average user.
Tools like TinkerTool and other third-party configuration tools put a GUI in front of many of these options so the need for using the defaults command is lessened.
Another way of changing how OS X operates is with the Option (or Alt) key. There are a lot of hidden nooks and crannies in OS X where the Option key will deliver some neat extra functionality, without obscure command line tool usage or third-party software.
Clicking the volume indicator on the menu bar presents you with a slider for volume. Clicking the volume indicator with the Option key held down lets you immediately assign what input and output devices are in use. Great for when you want to switch from the primary speakers to a headset when using a VoIP application, all without taking a trip to the System Preferences.
The Time Machine menubar icon allows you to enter Time Machine or start a backup when clicked. With Option held down, you can also verify backups or browse another Time Machine disk.
The Bluetooth menu bar icon offers a fair amount of functionality when clicked. Holding the Option key expands this by giving some interesting information, such as the Bluetooth address of the computer and the option to run various Bluetooth diagnostics utilities and the Bluetooth Explorer tool.
The airport menu bar icon provides more diagnostic-type information when clicked with the Option key held down. On the currently-selected Wi-Fi network, it will display useful information such as the connection mode (e.g. 802.11n), what type of security is in place (such as WPA2 Personal or WEP), and other troubleshooting or diagnostics information.
The Option key also works with other keyboard shortcuts. For instance, the top row of keys on a Mac keyboard serves a dual role: shortcuts for things like dimming the screen, playing music, adjusting volume, and so forth. If you push the Option key along with the short cut key, it will open the System Preferences for that action. For instance, the F3 key launches Expose to show all windows on the screen at once, and pushing Option+F3 will launch the Expose pane in System Preferences. Option+F10 will load the Sounds preference pane, and so on.
A lot of these hidden options won't show up in pull down menus or other traditional places. Experiment with clicking while the Option key is held down and you may find some hidden functionality tucked away in the operating system itself, or in various other applications.
Being a keyboard junkie, I love figuring these things out. Another extremely useful tool I keep in my toolbox is an application called KeyCue. While it won't help you figure out how the Option key may change certain clicks, it will display a list of all available hotkeys (global and specific to the current application) if you hold down the Command key for a few seconds. If you believe that keyboard shortcuts boost productivity, the cost of KeyCue will be worth it; it will help you learn keyboard shortcuts that you may not even know are available.