You can test websites and apps in all of the most common browsers (IE, Chrome, Firefox, Safari and Opera) and in all of the common versions (including both older and the newest versions), all without installing anything.
Source: Smashing Magazine: http://us1.campaign-archive.com/?u=16b832d9ad4b28edf261f34df&id=65d43ef796&e=c5b5dc3daa#a01
