TinySpell

Don't want to fire up your word processor just to spell-check a word? TinySpell will check your spelling in any application. A system tray icon will turn yellow if a word is misspelled, and will give you the option of correcting it. You can also add words to the dictionary. There are a handful of utilities out there that do the same thing, and none really stand out significantly. I like TinySpell's small size and ease-of use.

http://tinyspell.m6.net/