Flyspell
I’ve been so enjoying on-the-fly spellchecking in Safari that I wanted it in Emacs, too. Flyspell was mentioned in an aside in a MacDevCenter article, and I have it running now. It took me a while to figure out how to set up the hook to run it automatically (because I forget geek stuff faster than I learn it) - I found some examples in people’s on-line .emacs files: Thomas Krennwallner and Anoop Johnson.