There are a couple of web applications out there that allow to test a website in different browsers. I think that is a service the vendors of all theses browsers should pay for, but anyway:
- Browsershots.org: Free but you need to wait a long time to see results.
- Browserpool.de: German; Uses remote access, so you can test actual functionality (30€/month)
- Browsercam.com: a site to take screenshots of a page with many different browsers.
- SiteVista.com: Another site to take screenshots with a slick UI.
- SafariTest: Free, but only takes screenshots with Safari (but these are whole pages – not just the window). They offer a VNC testing for the Safari browser.
Given the pricing I think the best way would be to install a virtualizer/emulation software (like Parallels) and really run the tests off your own computer.