Everyone knows that you can execute the default (highlighted) action in a dialog (Cancel or OK) by pressing the Enter or Return keys. But did you know you can hit the first letter of a non-highlighted button in many programs to activate that option?
For example, in iTunes if you Command-Delete a song to delete it, you'll be presented with a dialog box confirming that you want to delete the song. The box has three buttons: Cancel, Keep Files, and Move to Trash. The Keep Files option is highlighted, but if you hit the M key, it will move the selection to the trash. It doesn't work in all apps, but it does for many.
[robg adds: This is different than a prior hint, which noted that there are command-key shortcuts for some unhighlighted selections -- Command-D for Don't Save, etc. Beyond iTunes, I'm not sure where else this works. I tested Safari and TextEdit and Mail...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060623043905816