There are some great little sites that have brilliant tools to quickly and easily help you develop and enhance your CSS 3 stylesheets. A list of those I know about right now are below.
css3.mikeplate.com For interactive css 3 code generation, this site does it all. Simply point, click and edit the value you want to change and see the demo display immediately take effect. The CSS 3 code needed is then shown at the bottom of the page. There are even bars to drag left and right to edit attribute settings.
border-radius.com A one-page interactive page that will generate the code needed to produce a rounded corner for one, two, three or four corners of your box.
css3please.com A niche site that allows developers to edit browser sensitive CSS 3 attributes more easily. When you change a browser sensitive attribute in their display page, the code is automatically updated to correct for other browsers.
CSS Lint Though this isn’t CSS 3 specific, I just had to list it anyway. Like it’s cousin JS Lint, it is an automated too to help you find inefficiencies in your code. I love automated tools that do work for you! The rules it gives you are sometimes general don’t have to be followed like gospel, but it does help.