OS X - Option-click Sound icon in the menu bar to change sound output and/or input quickly.
iOS - autocomplete text has the same typeface and style as the underlying text field
/via Idan Gazit
iCloud.com - Removes the spaces from the email input field when you go to the password field.
/via Andrey Usoltsev
Facebook - “Deleting” a comment youaccidentally posted a few seconds ago brings it back into the input box so you can try again.