When you are working on a new design for a website you want your design to look good in all browsers. Best is to test on:

FF - Firefox
Google Chrome
IE6 - Internet Explorer 6
IE7 - Internet Explorer 7
IE8 - Internet Explorer 8
IE9 - Internet Explorer 9

Most people do not have IE6 and IE7 installed anymore. IE7 can be emulated using compatibility mode in IE8 but IE6 usually is a problem.

With the following software (IETester) you can easily test your design in all versions of internet explorer:

My DebugBar | IETester / Browser Compatibility Check for Internet Explorer Versions from 5.5 to 9

It's a perfect IE9 IE8 IE7 and IE6 emulator and you can easily see your designs side by side.