Browser Emulator

With so many browser and so many versions out there, it is important to test a website in as many browsers as possible.

Currently, there is IE 8.0.6xxx but many users are still using IE6 and IE7, as well as Firefox of course.

What is the best way to test your website on all these browsers if you want to test it on your machine and don't have all these browsers installed? I have used Microsoft Expression Web SuperPreview to test between IE8,7,6 but the results for IE6 are not correct. When I test out website on a real IE6 installation (fortunately, we have someone int he office who still uses it), our website looks different than is shown in Microsoft Expression Web SuperPreview (which claims that it emulates IE 8,7,6 - no, it does not!)

So, my question is. Which tool, or website do you use as a browser emulator? Any tips are highly appreciated.

Regards,
Pascal

There are two possibilities for me at the moment:
1) My favorite: http://www.xenocode.com/browsers/
2) Another program for IE: IEtester http://www.my-debugbar.com/wiki/IETester/HomePage

Great!
Thank you for the fast reply.
http://www.xenocode.com/browsers/ didn't work for me on FF. Maybe I have to restart first after installing the .xpi

But IEtester http://www.my-debugbar.com/wiki/IETester/HomePage is exactly what I was looking for. Thanks a lot!!!