Global keyboard shortcut to switch to an application (to use with Emacs/Aquamacs)
I tried to use specifically Applescript/Automator to avoid 3rd party applications, but eventually I utilised capabilities of application called Alfred (I can not recall right now, but there is a chance you should buy Alfred Power Pack to use this feature)

In addition with the help of Karabiner.app I forced CapsLock button to act as Shift+Cmd+Ctrl+Opt and there is almost no need to use Opt+Tab to switch between apps since then.