How can I test my site in IE6, IE7, IE8, IE9, IE10 and Internet Explorer 11
Hitting F12
on IE8 it should start the Developer Tools that allows you to emulate IE7 (not IE6) using the Browser Mode.
I suppose on IE9 you will be able to emulate back at least until IE7, but I'm only supposing because now it irritates me the simple idea to have to buy W7 in order to test this.
UPDATE: as specified by Jeff Atwood: IE9 emulates back until IE7 too.
UPDATE 2: as suggested by Nick in comments below, if you want to be 100% sure to emulate the old IE browsers you can use VMs provided by Microsoft (incredibly they are also provided for free). As a side note, keep in mind that testing on VMs is more time consuming than using IE Developer Tools, IMHO VMs testing might be worth when testing something that's JS/CSS greedy like a web app or a complex website, not for simple sites that use jQuery and some CSS.
IE Tester It does IE6, 7, 8 & 9
Direct from Microsoft for just this purpose:
Internet Explorer Application Compatibility VPC Image