Worth checking out - currently in free open beta - is the new multi-browser preview suite from Adobe. It runs inside your normal browser using Flex, and allows any live URL to be previewed in a range of browsers under a range of OSs, including really cool things like onion-skinning each version to see the differences.
Each page is rendered as a screenshot-style image (so links, animations etc. don't work) but unlike many of the other "screenshot from many browsers" online services, BrowserLab understands Flash content and the various available OS typefaces, so will include that stuff in the render - you can even see if the font metrics for Safari under OSX are different to those in IE7 under Windows, and if that breaks your carefully-created layout.
http://browserlab.adobe.com/
Scott Fegette has an intro tutorial on AdobeTV explaining the system.
Adobe will be adding more browsers and OSs over time, and it may turn into a paid-for service, but at the moment it's free to use - just login with your Adobe ID, or create one at the start page.
