IE Spell

ieSpell is a free Internet Explorer browser extension that spell checks text input boxes on a webpage.

It should come in particularly handy for users who do a lot of web-based text entry (e.g. web mails, forums, blogs, diaries).
Even if your web application already includes spell checking functionality, you might still want to install this utility because it is definitely much faster than a server-side solution. Plus you get to store and use your personal word list across all your applications, instead of maintaining separate ones on each application.