BrowserLab Live on adobe.com
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