One of the most frustrating things that I've found in developing/debugging in IE is that you can only view the page source which was initially rendered with the first page load. This was acceptable a couple of years ago, but is really a huge limitation utilizing AJAX (and I'm not just referring to UpdatePanel's).
Of course you can view generated source with Firefox but what are you supposed to do when debugging any newly inserted client side script and/or viewing dynamically added markup? After searching around I found that you can simply copy and past the following script into the browsers address bar and it will open a new window with the pages currently generated source.
You can also add this to your IE favorites or add it to your Links toolbar so that its always one click away.
I cannot tell you how much this saved me this last week when I was using eval() to dynamically add script elements to the page's header.