I'm a fan of IEView and FirefoxView, two Firefox extensions that let me Right-click in Firefox and View in IE or Right-click in IE and click View in Firefox. They are very complimentary and have served me well.
Today both MikeG and Lifehacker pointed me to this stunning gem: Internet Explorer in a Firefox tab! I feel dirty already. What kind of world do we live in that creates such mutant children? What price my soul?