One of my favorite keyboard hacks was the ability to change the sending email address from OS X's Mail.app without resorting to
the mouse or trackpad like some farm boy fresh off the turnip truck. This shortcut existed through Snow Leopard but disappeared with Lion. I'm happy to report it's back in Mountain Lion.
Specifically, you create a keyboard shortcut for use in Mountain Lion's Mail.app that fills in the sending email address of your choice. In order to create it, you need to use the exact syntax from the Mail.app and the a handy keyboard shortcut. (I use Control + Command + Option + a corresponding key in the top row, Q-W-E-R.) Once you've set this up, you can quickly change the sending email address for any message you write in Mail.app with a keyboard combination. This screencast demonstrates.