One thing I really dig about OS X is the build-in spell checking. You can activate it in nearly every input field (even in the browser) and it’s really helpful to eliminate typing errors. One thing that always annoyed me was the hassle when typing in more than one language, German and English in my case. Therefore I’ve been a regular user of the spelling menu and I was more than surprised to find the “Multilingual” menu item the other day. Selected it checks the input again every supported dictionary. This will probably cause some words to falsely pass the spelling-test, but so far it has worked pretty good for me.