Here are 10 CSS UI Kits will definitely help you work faster whether you use them for basic wireframing or a full design. Enjoy !

TopCoat

CSS for clean and fast web apps.

A Hand-Coded Designer CSS UI Kit

This free UI Kit is coded by hand using HTML5 and CSS3 so you can easily drop these elements into any project you’re working on. This code is made to take advantage of modern browsers and to degrade gracefully in older versions of internet explorer, so no matter what your client or visitor is using they will be see an excellent design.

Icon Deposit CSS3 UI Kit (PSD+CSS)

This includes everything from GUI switches and buttons, to radio buttons and checkboxes. I also included the Photoshop PSD file for those who want to edit the design or make the images bigger.

Gumby

Gumby Framework’s UI Kit includes precooked buttons for you to use in your project’s UI. All you have to do is change color values to fit your site/project and you’re on your way to clickable button glory.

Bootstrap

Sleek, intuitive, and powerful front-end framework for faster and easier web development.

Lion CSS UI Kit

For web designers wanting to prototype native apps for Mac OS X Lion.

ui.css

ui.css is a tool for creating clean user interfaces for the web. The download includes a css file with styles for all the elements, including :hover and :active states.

Grid UI set

awesome little ui kit by designer Sven Read.

UI Kit

The link will take you to the comment where you can download the UI kit. The kit is almost entirely CSS (only two small images are used)

Light UI

Light UI Pack is specially for modern web design. It made with pure css3 and works in all modern browsers.

Designer CSS UI Kit

This UI kit has been designed and hand-coded from scratch in HTML5 taking advantage of modern CSS3 techniques, it includes a range of common user interface elements and font stacks, which look beautiful in modern browsers and degrade gracefully in Internet Explorer.