This is possibly something that I should have discovered quite some time ago, but I just stumbled on it today: enabling auto spellcheck in various applications.
I've been using OS X like a fiend the past 2-3 years or so, and I'd consider myself to be fairly advanced at this point. I've used spell check frequently before via the highlight -> click method, and silently griped about a lack of auto spellcheck in Safari's text fields. Well, looks like it was just one click away the whole time. Just right click on a text field in any application, go to "Spelling", and select "Check Spelling as You Type". Love it.