Spellify is a nice Ajax powered spell checker for form fields. Spellify will make it easier for your visitors to spell check while filling the text fields.
It suggests automatically if you misspell words in text fields, it shows correct words in a small box. Password fields are automatically ignored, you ca even specify if you want to ignore any text fields.
It works with major browsers like IE7, IE6, FF2, Opera 9 and Safari 3.