Browsershots: A Cross Platform Browser Test Tool

This is a terrific open source tool for Web designers: Browsershots (Inactive as of 2011)

Browsershots makes screenshots of your web design in different browsers. It is a free open-source online service created by Johann C. Rocholl. When you submit your web address, it will be added to the job queue. A number of distributed computers will open your website in their browser.

They make screenshots of your site in all the browsers you select for Linux, Windows, Mac OS, Gecko, and KHTML/WebKit and upload them to their central server.
Additional conditions you can specify are screen size, color depth, Javascript, Java, and Flash support. When the screenshots come up, you can then hover over each screenshot to see a larger version or click on it to see more details. You can also download all the screenshots with one click!