I’ve just read on Ajaxian about a jQuery snippet of code that checks a page you are working on has any CSS float errors. It highlights the elements that should have been cleared by putting an orange dotted browser around them.
I’m sure this will come in handy as its one of the major issues when dealing with cross browser compatibility issues.
The site needs to have jQuery installed for it to work. You can add it as a bookmarklet if you wish by dragging this link to your links toolbar.