I’m probably a bit behind the times with this one, but Adobe have added “BrowserLab” functionality to “preview and test your web pages on leading browsers and operating systems – on demand“.
Seems to work like other paid services, but is free 
Available browser options (across mac and windows) include:
- IE 6/7/8
- Firefox 2/3
- Safari 3/4
2 Comments
Please check http://www.browserseal.com/ – although it supports less browsers, it can easily beat BrowserLab in terms of speed.
This is another nice option:
http://www.spoon.net/browsers/