I have just discovered that Jetstrap is a 100% web-based interface building tool for Twitter Bootstrap. No software to download, just log in to Jetstrap and build. Your work is accessible from anywhere. Built for developers, designers, and idea-people alike, Jetstrap helps you get awesome websites up and running fast, with less work and digging through docs.  Drag and drop buttons, Jumbotrons, Images, containers, lists, navigation bars and many more which can be customisable. Try the demo and you will love it.

Check out Jetstrap for superb prototypes.