I've read about that tool at 37 Signals and watched the short demo. I should say it looks brilliant. Quite obvious for Mac users but it is first time I see such a good implementation on Windows.
In fact, they put to life "Command" button inspired by Jeff Raskin in his The Humane Interface book. The implementation is simple, and very impressive - they just use "Caps Lock" key as such "Command" button.
And looks like Aza Raskin (who is one of the owners of Humanized) is a good son of his father.
Well, I'd like to see Linux port of this tool. Anyone?