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).
Laura Gibbs, of XPLANA points to this Internet Explorer extention for Windows users that adds spell checking cabilities to browser.
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]