If you are designing a website or a website template or even blog themes, it's essential to test your site in all major browsers to ascertain that they are rendered properly. Major browser includes Internet Explorer, Firefox, Opera and Safari. Usually, the one which cause trouble is Internet Explorer and since the majority of users use IE it becomes necessary to make sure your site displays correctly in the IE engine. Now Windows doesn't allow you to install different versions of IE on the same machine (there are probably some hacks available for this, but I'm not sure about them). If you ever have to test your site on different versions of IE, then use the following tools.
Image courtesy: BetaNews
If the bugs bother then you can download the Internet Explorer Collection, which is a pack containing standalone versions of IE right from version 1.0 to 8.0. They can be run all at the same time.
Update: The Internet Explorer Collection pack has been renamed Utilu IE Collection
Another useful online service called Browsershots allows you to test how websites look on different browsers. Browsershots takes screenshots of your website in different browsers running on different operating system. You choose the browsers you want to test on, submit your site and wait till it processes your request. Your website will be opened on a number of computers, a screenshot taken and uploaded back to the server. A very useful tool for weeding out design problems.