If you're an XHTML and CSS designer you've had the problem where your beautiful design breaks in different browsers. Therefore, you have to recheck your designs in the major browsers. I normally go with IE6+ and Firefox 1.5+.
Due to the huge number of different browsers out there on different Operating Systems, it usually isn't possible to have access to each browser/OS configuration. That is why many designers use as services such as browsercam.com; which is a paid service that generates screenshots of web pages in different browsers running on different operating systems.
Today while looking for alternatives I stumbled upon http://browsershots.org/. This is an Open Source service that takes screenshots of web pages in different browsers and Operating Systems on distributed computers working together via XML-RPC. Contributers to the service register as Screenshot Factories which poll the Server's for screenshot requests in its queue.