I found CocoAspell. It's a Cocoa frontend for Aspell, a UNIX spell checker. Seems to work very well, lot of different language dictionaries available, too.
[Editor's note: I realize that there's built-in spell checking in many (all?) Cocoa apps, but CocoAspell has some nice features, including filters that skip things like HTML, URLs, or email addresses. It includes a preference panel and a service which are easily installed with a couple of drag and drop operations. To see it in action, you need to choose the Spelling menu in a Cocoa app and then point to the Aspell dictionary.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020425003659789