What really got me thinking about this recently was my interest in weaning myself away from table-based design in favor of more CSS-driven design. I know it's supposed to be so much better, the wave of the future and all that. But my concern has always been how to be sure it would work on older browsers.
I try to keep the latest versions of IE, FireFox, NS Navigator, Opera and now Google Chrome on my local machine for some basic cross-browser testing. But I can't have the other versions or the other O/S configurations to test. I don't even own a Mac at present.
I started thinking it would be cool if there was a software tool out there that would grab a page and render it just like a specified browser, version of browser, etc.
There are a few restrictions on the "free" part. You can only stay logged into a session for 5 minutes at a time. But you can launch as many sessions as you like. So for quick tests, it is perfectly adequate. Also, paying customers get preference for access when the site gets busy. If you need to do more complex testing and need more than 5 minutes on an image, you can buy little blocks of time. It's all very well thought-out.
Now, if they could just do something similar to test wireless devices...