Since you ask for a plug-in I assume this is a Web application. I don't know of any offhand, but a search of the R4/R5 forum on LDD yielded the following, which mentions JSpell (though not in a positive way), Hot Lingo and Spellchecker:
Dig Deeper on Lotus Notes Domino Application Development
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.