gSpell is a service for Mac OS X that provides spelling suggestions from Google. For some words and phrases Google's spell checker is much more accurate than a desktop spell checker (since their sites generate insane amounts of statistical data regarding the world's most common spelling mistakes [searches] and their intended spellings [results clicked]). Their spell checker also corrects formal names (can your spell checker fix Rachmananoph, Seve Jobz, or Afex Twen?) and colloquialisms not yet included in desktop spell checkers.
gSpell was developed as a final project for the Search Engines: Technology, Society, and Business seminar taught in Fall 2005 at UC Berkeley. It runs on Mac OS X 10.4 or better.
A screencast: