ui.css
Metro UI CSS
Pure CSS
jQuery jKit
Solid HTML5/CSS3 UI Kit
CSS3 UI Kit
Alt CSS3 UI Kit
MediaLoot Designer UI Kit
Flat Free
Futurico UI
Designmodo/Pixeden Web UI Set
Red Pure CSS
Light CSS3 UI Kit
Simple UI Kit
Lion CSS UI Kit
Eureka UI Kit
Yet Another UI Kit
Dark UI Elements
Flat UI Kit
Coconut UI CSS
via spyrestudios
https://www.oschina.net/news/42379/20-free-open-source-css3-user-interface-kits