I’ve seen Project Comet in action when I’ve visited Adobe Head Quarters last summer and now it is called Adobe XD. It’s all about designing a prototype for your website over a variety of devices.
It works a bit like the app Proto from the past. But this app works only on Mac (for now) and gives you loads of cool tools and tricks to make your well designed prototype work.
Check it out now! http://adobe.ly/1QQASWt