I’m a Mac user, but this Firefox plugin is so slick, I’m actually envious of my PC based web developer brethren. It’s called IE Tab from PCMan (Hong Jen Yee). Once installed, you browse to any web page using Firefox. Then, simply right click to bring up the contextual menu and choose “View Page in IE Tab”. The same page loads in a new tab, but displays using Internet Explorer’s rendering engine.
Now there’s no reason to ever launch IE again! When I’m checking how a page renders on my test PC, I can simply launch Firefox and see both the FF and IE result.