OS X - Option-click Sound icon in the menu bar to change sound output and/or input quickly.
OS X - Searching in System Preferences highlights the relevant sections. The more relevant the section, the brighter the light will be.
OS X - Adjust the volume in 1/4 increments with shift+alt+volume keys.
OS X Mountain Lion - When you trigger the Speech & Dictation dialogue, the fans on your machine immediately go minimum speed to prevent background noise.
iCal - Shows birthdays from Address Book with the person’s name and age.
/via Clemens Mayer