I’ve seen others test out websites in IETester and IE Tab but these tools aren’t always 100% accurate. For instance you might forget to account for the Eolas bug that will not show up with IE Tab. Also IE6 sometimes does a shift and flicker when it’s rendering up a page, which IE Tester will not do, thus the page will look off in IE Tester but not in the real deal.
In my opinion it’s best to use the free virtual machines that Microsoft has for IE6, IE7 and IE8 beta 2. A few are on XP and another is on Vista, I recommend the XP versions, since they are smaller in filesize and will run much faster. The Virtual Machines expire after a few months, but so far Microsoft has been continuing to make new ones for free.
Converting them for use on VMware products is easy, but you do need Virtual PC to create the *.vmc file (this holds the configuration). Once you have the vmc file you need to place it in the same folder as the vhd file and then you can use VMware Converter to convert it into a virtual machine you can use in VMware Player or VMware Server, which are all free products.
Of course you could always setup an old Windows 2000 machine, since the last version of Internet Explorer it can run is IE6. And if you are stuck on a Windows 2000 machine, you could always use these virtual machines and install Chrome or Safari, since neither work on Windows 2000. However, I wouldn’t spend too much time configuring the free virtual machines though, since they expire every few months.