I've been using this tool for over a year, but only recently dived more into what it could do, and I continue to be very impressed. Beyond Compare is a file comparison tool above all others. Not only does it have the best name for a comparison tool ever, but it has all of the bells and whistles you could imagine, and a well-executed UI that keeps most of them out of the way until you need them. And I haven't even tried out the plug-ins...

It's well worth $30.