Holding ⌥ while (Option)while clicking menu bar icons will give you additional menu items or alternate menus. The alternate volume control menu will let you choose input and output audio devices.

Bonus: On older versions of OS X, this gave you the alert volume instead of devices. You can still get the alert volume using ⇧ clicking.
For the airport menu, you'll get WiFi connection information.

The BlueToothBluetooth menu will include debugging information and tools.

Time Machine will replace the normal operations withshow "Verify Backups" and "Browse Other Time Machine Disks". Unlike the other menus, the Time Machine menu will show the alternates if you hold Option after opening the menu, and will hide them if you release it.
