John Gruber points to gSpell
, a neat little OS X Service that leverages the power of Google as a spell checker. The author, Nathan Spindel, had the clever realization that Google's spelling suggestions are better than the built-in spell checker because their "sites generate insane amounts of statistical data regarding the world's most common spelling mistakes [searches] and their intended spellings [results clicked]." Using it is easy; just select the text and invoke gSpell through the Services menu or by keyboard shortcut.
gSpell apparently began as a programming exercise and is now available as a free download
[Via Daring Fireball