淘宝使用的灰色度方案

http://licai.taobao.com/

<style>
.fre-box { float: left; display: inline; width: 85px; height: 35px; padding: 0px 5px; }
.fre-box img { display: block; width: 80px; height: 30px; opacity: .5; filter: gray alpha(opacity=50); -webkit-filter: grayscale(1); }
.fre-box a:hover img { -webkit-filter: grayscale(0); opacity: 1; filter: alpha(opacity=100); filter: none; }
</style>