Just a few tools I've found quite useful in my (mac-based) workflow:
TextMate: A true productivity enhancer. Costs a few bucks but worth every penny. Be sure to familiarize yourself with all the shortcuts and available bundles as well as the trick for how to edit remote files.
CSS Edit: Another beautiful program. X-ray view allows you to really dig into your CSS and is very handy for troubleshooting.
Blueprint CSS: Well grounded CSS framework for straightforward, quick development.
Expression Engine: This CMS is wonderful. Simple to use, well documented, and great pricing. It has allowed me to focus on designing websites with XHTML and CSS, while simplifying the tools to create dynamic websites. Also, has a pretty user-friendly admin side. The forums are quite useful for any problems that arise.
Transmit: I searched long and far for an FTP program that has the same column view as Mac's Finder and Transmit is it. Great program all around.
Web Developer Toolbar: While there are prettier browsers out there to surf the web, if you develop websites Firefox is really the way to go. This extension adds a host of tools to your arsenal.
Firebug: This one adds more.
YSlow: And this one even more!