I recommend Ultimate CSS Gradient Generator – ColorZilla.com
It’s a well designed tool and there’s a white to transparent preset.
The CSS code that it generates for a white to transparent gradient is this:
background: -moz-linear-gradient(top, rgba(255,255,255,1) 0%, rgba(255,255,255,0) 100%); /* FF3.6-15 */
background: -webkit-linear-gradient(top, rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* Chrome10-25,Safari5.1-6 */
background: linear-gradient(to bottom, rgba(255,255,255,1) 0%,rgba(255,255,255,0) 100%); /* W3C, IE10+, FF16+, Chrome26+, Opera12+, Safari7+ */
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr="#ffffff", endColorstr="#00ffffff",GradientType=0 ); /* IE6-9 */