"A new major release of our good and widely known FCKeditor is available. We've been focused on fixing small compatibility issues with some browsers (IE8 mainly) and porting a nice feature introduced with CKEditor 3, our unique Spell Check As You Type (SCAYT) solution.
This release also introduces some important security fixes. Upgrading is highly recommended.

Spell Check As You Type (SCAYT)
This is one of the exciting new features introduced with CKEditor. A new spell checking solution that becomes usual to end users due to its usability. Misspelled words will be highlighted with a red line, having replacement suggestions when opening the context menu over them. We're know offering it in FCKeditor also.

This feature is available in partnership with SpellChecker.net for free. The free version actually presents a banner space with advertising, but requires no server installations to have in working, as long as Internet connection is available. It's an out of the box solution. A banner free version of it as well as locally hosted solutions are available under payment directly from SpellChecker.net."
(source CKEditor official site)