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.
What wins me over in this application is the ease and the speed of service. Have a look at this screen cast where I am comparing the twitter homepage. The screen shots load in a few seconds, you can easily compare the page in various browsers, it’s simple and perfect, at least for my needs, this is the ultimate cross browser testing tool.
You are free to visit the Adobe Browser Lab and sign up for a beta account.