A new project called Browsershots allows web designers to see what their site looks like in a multitude of browsers and platforms with a trivial amount of effort.
When you go to Browsershots, all you have to do is enter a website and check the browsers you want to test. It supports 31 Linux browsers, 6 Windows browsers, and 2 Macintosh browsers. These counts include both variants such as MSIE 6 and 7 as well as rare browsers such as Iceweasel and SeaMonkey.
A collection of computers running the various operating systems and browsers open the provided site and take screen shots. These are then presented to the designer, if he is patient enough to wait for them.
A serious problem with Browsershots is that it can literally take hours to get back all of the screenshots. There is a publically available queue to let you know where you are in line, but unless it is really short you should use the maximum timeout on your request. Currently that is 4 hours.
The service currently describes itself as a Technology Preview and is free of charge.