Web Design Tools that I can no longer live without

One of the biggest pains when building a new web site on MOSS is cross browser development and testing. Often, slight errors in stylesheets can have disasterous affects on different browsers. One of the tools that I have found help me in this regard is the FireBug add-in within Firefox. This tool lets you inspect a particular element on the page. It then tells you where it is picking up the associated styles for the selected element. It references each stylesheet name that has been included, the line number, and which CSS attributes are being ignored. It really helps you to quickly identify bizaar behaviours within a page.