- Exchange Web Mail in Vista (damn that red X!)
- Spell Checking in Textboxes on the Web
The 3rd reason is no longer a good one for me! Check out ieSpell. Judging by the interface this tool has been around a while but I have only just stumbled upon it. ieSpell also has some handy features like looking up words on any page in Wikipedia or Dictionary.com via a right mouse click, very nice.
One tip for Windows Vista users still putting up with UAC: When configuring ieSpell ensure that you launch the browser as an Administrator - if you don't it won't remember your preferences.
[*] Don't get me wrong; I don't dislike FireFox (or any browser for that matter) I just prefer IE and use it wherever I can.