The Flat UI HTML & CSS Kit is made on the basis of Twitter Bootstrap, a comfortable and functional framework that simplifies the development of websites. The framework has been proven over the years by developers from around the world.
Flat UI Elements PSD
Flat UI Pro is made on the basis of Twitter Bootstrap in a stunning flat-style, and the kit also includes a PSD version for designers. From $39. Learn More: http://designmodo.com/flat/