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.