Material Design for Bootstrap is a theme for Bootstrap 3 which let you use Google Material Design in your favourite front-end framework.
uilang is a minimal programming language for web designers. You can easily create custom user interface components and build prototypes.
Bootflat is an open source flat UI kit based on Twitter’s Bootstrap 3.1 CSS framework. It provides a faster way to create elegant web apps.
Here is a flat design user interface inspired to a PSD freebie by GraphicBurger. This HTML5 + CSS3 ready-coded template is not complete yet.
Responsive Flat UI kit is a ready coded HTML custom UI set built to work with and sit next to Bootstrap 3. Created by Catalin Rosu.
Flat UI Free is an awesome HTML UI kit containing a lot of useful web elements for your web projects. It has been made with Bootstrap.
A feature-packed user interface kit containing buttons, input fields, sliders, progress bars, and various other form elements. It’s also fully coded for you in CSS and HTML.