For me this is a big problem because half my visitors to my several sites are still using IE6 and you have to admit that IE6 is a pathetic browser.
IE7 is a lot more compliant and displays sites a lot correctly than its predecessors.
While there are several services available, one service I love to check what my browser looks like is IE NetRenderer.
IE NetRenderer allows you to check how a website is rendered by Internet Explorer 7, 6 or 5.5. It displays the screenshots in full length along with small rulers which you can use to check your site properly.
You also have a differences mode which gives you an exact understanding on how differently both versions of the browser display your page.
The images are rendered quickly. The bottleneck you will face is downloading the image (for users on slower connections) to actually preview this.
IE NetRenderer is a real life saver especially when I want to check if that particular CSS hack will actually work or not!