While searching for a keyboard shortcut in OmniFocus that will "Focus in a new window", I came across this cool widget called Dashkards.
Dashkards is a widget that shows up in your OS X Dashboard. It's a handy little sheet that displays the keyboard shortcuts for a given app. There are a lot of other supported apps like OmniFocus, Acorn, Evernote, Scrivener, and even Markdown.
And by the way, while there isn't a keyboard shortcut for "Focus in a new window", alt/option double-click on an action will do the job.