From browsershots.org:

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. Then they will make screenshots and upload them to the central server here.

Just go to browsershots.org and specify which OS/browser combinations to use.

Supported OSes

• Linux
• Windows
• Mac OS
• BSD
• Gecko
• KHTML/WebKit

Supported Browsers

• Firefox
• Internet Explorer
• Google Chrome
• Opera
• Netscape
• Konqueror
• SeaMonkey 
• Many more 

Misc. Options

• Screen size
• Color depth
• Javascript
• Java
• Flash

Examples from browsershots.org:

Firefox 3.5.7 / Ubuntu 9.10 (Karmic Koala)
for http://www.powercram.com/