Pure

Simple markup, lightweight and made by Yahoo!

Twitter Bootstrap

So popular it has it's own themes.

Fries

Lets you create sexy Android-like UI.

Skeleton

Responsive framework with a grid, typography, buttons and form styles.

Foundation

The most advanced responsive front-end framework in the world.

LimeJS

HTML5 game framework for all modern touchscreens and desktop browsers.

Metro UI CSS

Theme looks like WIndows 8 and is styled with LESS.

Maxmertkit

Styled with SASS and built with node.js

Cool Kitten

Parallax scrolling responsive framework.

uikit

A lightweight and modular front-end framework for developing fast and powerful web interfaces.