It’s always fun to stumble across a little-known feature that’s been around for years. When you’re typing text in many OS X applications (Safari, TextEdit, Mail, etc), you can press ESC (F5 also works) for an pop-up list with words from the built in OS X dictionary.
Note: This trick only works in Cocoa apps. It works in most of Apple’s built-in software, but unfortunately not in many Adobe or Microsoft applications.
Source: We’ve run into this tip on a number of blogs over the years, but when John Gruber of Daring Fireball singled it out as one of those great tips many Mac users don’t know, we knew it was time to include it in our collection.