ieSpell - Spell Checker add-on for Internet Explorer
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).
For Mac OS X users, Safari 1.0 has the ability to spell check web forms. The tool is off by default and needs to be enabled. To do so, open a web form page, insert the cursor, then go to the Edit menu and choose: Spelling > Check Spelling as you Type... [by way of...Steve Gillmor's Emerging Opps]