Somehow nobody has mentioned the easy, no-buying-an-MS-OS-way of testing in IE on Linux and OS X - IEs4Linux.

Have a look at the installation instructions: http://www.tatanka.com.br/ies4linux/page/Installation

It is very easy to set up and allows you to run IE 6 and 7 without installing Windows on anything.

Don't let the name put you off, it also runs on OS X (http://www.tatanka.com.br/ies4linux/news/49).