Option-click menu extras to reveal hidden settings
Menu extras are the little icon menus that live over on the right-hand side of your menu bar. Most people will probably have the Volume and Airport menus, but there are loads more that you can enable.
You can find 26 built in ones located in /System/Library/CoreServices/Menu Extras that you can just double-click to start. On top of this there are many more that come with applications (like Evernote, QuickSilver and Tweetie) and some that are small applications themselves (like Caffeine, F.lux and iStat menus).
If you have a lot of menu extras, you can rearrange them by holding the command key and dragging them around. To remove one, just hold command and drag it off the menu bar.
One not so well known trick is holding down the Option key when clicking on the menus. For some of them this will reveal hidden settings and information that aren't normally shown in the menu. Here's what you'll see in the Option-click menu for some of the most common ones.
In Snow Leopard the Airport menu has already got a lot more informative with the new way the icon displays the current status. In the Option-click menu you'll see a lot of extra information about your current wireless network, including the protocol (e.g. 802.11n), the signal strength (RSSI), security mode (e.g. WPA2 Personal), the channel (e.g. 11) and some other things like the hardware address and the transmit rate. If you want to get some of this information about another wireless network in the list that you aren't using, just hold your mouse over it and a yellow tooltip will appear.
This menu normally just shows a slider for you to adjust the volume, but option-clicking shows some settings that you can normally change in the Sound section of System Preferences. It allows you to select different input and output devices, such as an external microphone or external speakers.
If you have Bluetooth on your Mac, option-clicking the menu will show you the bluetooth version number. If you have the developer tools installed, it will also give you options to open Bluetooth Explorer and Packetlogger.
This one is just for laptop users. Option-clicking will add an item at the top of the menu that displays the condition of your battery. Clicking on this will open up a help page explaining what your condition means. You can find out more information about your battery here.
A normal click on this menu gives the time of the last sync, but option-clicking gives a huge list of the last time each individual data type was synced (i.e. Bookmarks, Calendars, Contacts etc.). It also adds in an option to open Sync Diagnostics and iSync, as well as giving the option to reset sync services.
In this one, Option-clicking changes "Enter Time Machine" to "Browse Other Time Machine Disks". This brings up a dialog listing all the connected drives that contain Time Machine backups. Select one and click Use Selected Disk to browse the Time Machine backup.