Fluid UI - app design tool

From the always excellent iOS Dev Weekly mailout:

Fluid UI

This is a slick, web based prototyping and wireframing tool for building quick iOS (and others) app mockups. Feels a bit like the Xcode storyboard editor but implemented in a web browser.

Wow. How did I miss this? Very useful, and the default template doesn't look like the iPhone, which I think is better for a quick drawing tool.