I stand by the fact that one the best tools available for SharePoint Customization is the Internet Explorer Developer Toolbar. This tool has recently seen a new version released (BETA3).
The IE Dev Toolbar let’s you do things like quickly view your pages in different resolutions, outline divs, images, or table elements. One of the more useful features allow you to inspect the DOM by clicking on an element on the page to see what properties are applied to it, things such as what CSS class.
For those that have asked, when testing pages in Firefox, I use Firebug another very useful tool.