A rich text editor in Flash
The second after I saw Flash 6 for the first time, I thought it was going to become a GUI for applications, before the Macromedia rep. started to talk about RIA. Two seconds after that, I thought about exactly this.
I've become highly frustrated by the poorness of text editing features that you find in today's CMS, from the free blog to the zillion bucks system. When they have something, it's always limited to Windows if not just MSIE on Windows.
So am I adding my voice to many others: a rich text editor in Flash is what we, content workers, need. And it's so insanely matching our needs with the possibility to get a cool UI that works with our computers of choice (or anything that runs Flash), that this is an obvious killer application.
As for the price, anything that handles simple XHTML with the mere B (strong), I (em), links and image must be free. I'm ready to pay for something that nicely handles my structural XML tags described in my schema or DTD. Oh, and I want drag-n-drop and offline editing with that :).