It's extremely helpful as a web designer to be able to check compatibility across all browsers without having to install them. Installing IE6/7/8 on the same machine is not recommended.
I've found http://browsershots.org/ to be useful for that. Of course, you can't actually use the site through it.
It might just be me but their processing times seem to have been growing and growing. Also I've been getting frequent erroring machine s(screenshots obscured by dialogs etc.). Might just be my luck though.