How to tab between buttons on an Mac OS X dialog box

If you enable Full Keyboard Access by setting it to all controls, you should be able to tab between options and select by hitting spacebar, at least for OS X 10.5:

While @ruddzw's (chosen) answer works, it can slow you down in other areas simply from the sheer number of controls there are to tab through.

@NReilingh presents a different approach, and the semantics behind it:

The right way to go about keyboard shortcuts for alternate dialog buttons is not tab, but command key combos. For example, Command-period is almost universally "cancel." [...]
It's often not documented, but the command shortcut for different buttons that aren't the default button or cancel is usually the first letter of that button.

So in the example:

• enter/return will Save,
• ⌘+. or esc will Cancel, and
• ⌘+D will map to Don't Save.
  As @Kevin Reid points out, ⌘+delete works here as well.

In Keyboard preferences under Keyboard Shortcuts

Full Keyboard Access: Switch to All Controls instead of Text Boxes and Lists Only