Unfortunately, the options are somewhat limited.
You can use VMWare with Windows XP installation (or lower) to test IE6
You can use VirtualPC (free) with (free) Microsoft XP / IE6 image
You used to use Spoon.net to launch only the browser through the plugin in Firefox / whatever
You can try MultipleIE, but IE6 will break and not work if IE8 is installed locally
Or, if you're really lucky, you can support IE6 DROP!
source share