Groundwork is a fully responsive HTML5, CSS and Javascript toolkit. Using Groundwork, you can quickly build web apps that work on virtually any device.
Cool project I saw on Github, though I don't think it has any functionality that Bootstrap doesn't (and I wish they used
hint.css for their tooltips instead of using js). The site does have built in resizing to show you how it actually works, which is something the bootstrap website could definitely use.