We love rapid prototyping at Storm. Firing things together quickly brings our clients closer to the development teams and lets us make sure the product is exactly what they wanted.
The bootstrap gives you a CSS file you can easily include on any project, even by hot linking to their URL on GitHub (if you’re comfortable knowing things will break should GitHub go down!) and provides a grid layout system, along with styles for all the common elements used on a website. From tables, divs and HTML5 elements; they’re all styled for you out of the box in a simple admin interface kind of way.
They even provide source code as jQuery extensions for making modals appear, and HTML Table sorting.
Their documentation on GitHub is also excellent, and explains the grid layout system easily with lots of examples, which makes life easier for me coming from a background of preferring to roll my own code rather relying on frameworks.