Everyone knows how much of a pain it is to debug and test stuff in IE - especially for cross browser compatability and JS/AJAX errors or callbacks.
For development purposes - the following company provide a few nice solutions that could help with developing web applications in a number of versions of Internet Explorer.
DebugBar DebugBar: Is a toolbar which offers HTML DOM Tree view, HTTP and HTTPS Headers, page cookies view, HTML validation, JS Methods view, JS Execution etc. for Internet Explorer.
Last but not least...
These are probably worth checking out as a solution to aid development and testing within Internet Explorer.
Microsoft (the King of browsers) have released the Beta of a new Cross Browser Compatability tool which allows you to compare different rendering engines - named "Expression Web SuperPreview".
As an alternative to having Virtual Machine software with different environments configured - or Multiple IE's installed, this looks, on paper, to be a sound solution.
Details can be found here: Expression Web SuperPreview.
Sean Corfield has blogged about a new unit testing framework which integrates with eclipse.
You can check it out here: MX Unit
Well, not so new, but version 1.0 of Coldfire has just been released.
It plugs into the Firebug Firefox extension and works a treat.
Coldfire plugs into it and displays all the usual debugging information in a much better format.
It also allows you to watch ColdFusion variables as the request is executed.
More information and Download