Did you ever have problems to test your javascript or stylesheets on different browsers? Thhe most simple answer is to install Firefox, Opera, Safari, etc on your development computer and use system Internet explorer if using a Windows OS.
For Firefox, Opera, Safari and browsers like these testing with latest version has an acceptable result because most of people using them are get informed about new versions and they'll upgrade to latest version within one month at the most latest time.
But testing in IE is really confusing. IE6 and IE7 are using rendering and javascript engines with too much difference also still there are too many users rather to use Windows XP and too many of them rather not to upgrade their IE browser to version 7.
What's the solution?
1. Having two PCs one with original Windows XP's IE6 and one with Vista's IE 7. But it costs and also its difficult to work on two different PCs for example if you work based on a server located on a virtual machine on one of them!
2. Using a virtual machine to install another Windows with different version of Internet explorer. But this also has some difficulties!
3. USING IETESTER. Cool, yeah! I found this cool program lately and as they said in their website it's a FREE web browser that you can use IE 5.5, IE6, IE7 and also IE8 beta javascript and rendering engines at same time in just different tabs!. Great idea, great program for web developers but still it has some problems. They talked about some of these problems in their site but I found some too for example it takes too much time to reach a successfull AJAX request but who cares it saves us so much time by leaving PC changings or virtual machine startups and let it retake some on AJAX requests. :D
Anyway, if you like to download it please go to http://www.my-debugbar.com/wiki/IETester/HomePage and also donate if you can.