ieSpell

From Red Egg Software:

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.

The program installs as a new button in the IE toolbar (as well as a new menu item under "Tools") - after filling in a form, just hit the ieSpell button and it pops up a dialog, similar to the MS Word spell check. ieSpell also works (right-click menu only) on other IE based browsers such as CrazyBrowser, MSN, MyIE, MyIE2, NetCaptor, etc.