The chrispederick.com Firefox Web Developer’s Extension. It’s a suite of tools to help you develop CSS style sheets. You can also use it to play with other people’s sites when you visit them (of course you can’t save the changes on their site, it’s just for your machine). It has serious WYSIWYG capabilities, because you can just sit there tweaking it and see the changes right then and there. I’d say it works equally well to Dreamweaver’s CSS preview.