I just came across <spellcheck:*>>, a neat replacement for the TextArea and RichTextArea components. It provides client-side or server-side spell checking, along with custom dictionaries.
The best part is that it’s donation-ware, so just claim you built it yourself, it took a week, and pass your hourly rate along to the author*. Just don’t send them $5 and consider yourself even 🙂
*Don’t actually do this.