A good while ago, I posted a little review of BrowserLab, a tool I believed was irreplaceable to any template coder. I was wrong.
I have been using The Adobe Browser Lab for quite a while now and sincerely cannot fault it. It allows you to compare screen shots of your web sites side by side, in onion view and in plain site view. You can zoom in on the screen shots, set capture timeouts and do many other cool things.