Now and then you may have a client that users a combination of OS and browser that you cannot test a site design on. I found this great resource that provides screenshots from browsers on different platforms.

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. – Browsershots Site

This save having to get a Mac and a Linux box just to test all the browser versions and how they render the site. Of course this doesn’t allow you to actually test how the site operates on these browser, but at least you can see if something is out of place.