Open source plug-in for WordPress and TinyMCE that spell-checks, grammar-checks, and style-checks, and provides explanations for errors. In addition to English, the spell-checker is available for French, German, Portuguese, and Spanish. URL: After the Deadline
Spell check your entire website in one go. URL: Spellr.us.
Automatic spell checker that displays misspelled words and their suggestions soon after you stop typing. URL: Spellify.