Just found this great tool IEs4Linux for testing websites under Internet Explorer 6, 5.5 and 5 on linux. It currently doesn’t support IE7 but that feature is in development beta. Find more info here: http://www.tatanka.com.br/ies4linux/page/Main_Page
Update (Jun 23, 2011):
IEs4Linux now also supports IE7, IE8 and IE9 in beta.
Kartik - The TechGlider Guy