OS X has a built in spell checker that lots of programs use. When you're typing in Mail.app it will put a red underline under words it doesn't know. For ages I'd been wondering if you could add words into the built in dictionary. I happened to right click (ctrl-click with a single button mouse) and you can choose 'Learn Spelling' or 'Ignore Spelling'. Very handy. Blogging wasn't in there, but it is now.