One of the challenges of web development is the plethora of web browsers & OS combinations and the differences in rendering between them. You usually need to test your site in at least 3 to 4 “Level 1” browsers your work before releasing it wide. Beyond this number, it becomes expensive to maintain all the different hardware/os/browsers available. There have been commercial sites available for some years now offering screenshots taken in different browsers. Now you have a free version at use Browsershots.org.
The way it works, is it queues your job requests and processes them in multiple machines (shot factories). The best part is the entire source code that makes this work is available – so you can go check out how they made this whole thing work across platforms.
Update (15/Nov/06): A new site that offers limited (only till viewport) for IE 6 & 7 similar service is IE Web Renderer.