Browsershots.org is a free online service that allows you to test your web design in different browsers.
You simply submit your web site URL. A number of distributed computers automatically open your web site in a browser. They take screenshots of the site, and upload them to a page at browsershot.org. You can bookmark this page and return to it later, or send it to your designers or developers for their information.
It's a very useful service for web designers, not only because it aids the design process, but because it allows you to demonstrate to your clients the importance of compatible design. After all, seeing is believing!