http://bootstrapstudio.io/
Bootstrap Studio is a desktop application that helps web developers and designers create responsive websites using the Bootstrap framework. It is fantastically easy to use - just drag what you need and drop it on the page. Preview your designs as you make them in real time. Your websites work on every device - from a phone to a computer screen!
Haha Izzy, I assume you missed the text that was meant to go with this?
True. That was lazy of me... 
I've just had a quick play with the web demo of this, and it's very impressive. Quick, responsive, not batshit insane (more common in webdev tools than you'd think).
Only problem with it is that Bootstrap's still a bit heavy for mobile landers - but given how many sites use it these days, it'd be interesting to do an RUM test and see if it's nearly always cached locally anyway. Hmm...
Nice share!