View Rendered Source in IE
Rendered source chart in particular is a very slick plugin. It not only takes everything in the DOM and shows it as HTML, but it also colour codes and formats it so it looks all pretty like this:
Today I went looking for something similar for IE and came across a similar tool that works on IE. It’s not got as many features as the Firefox plugin (and the rendered source doesn’t look as nice) but it definitely made my debugging much easier because it meant I could cut and paste the current snapshot of my page’s DOM into a HTML file and play with it to get an idea of why it didn’t work.