Beta Tool CSSVista is a free application which allows you to edit your website CSS for style and compatibility on the two most used browsers, IE and FF, at the same time.
The only other live CSS editor I know of is the Mozilla Add-On by Chris Pederick called Web Developer - which I have used with great frequency and find exceptionally useful.
Litmus is the first I've heard which lets you do both together (sounds awesome!) so I am anxious to get started testing.
