Want tab to go to every field on a form not just input fields?
I got sick of the state/country drop down fields of web forms being skipped when tabbing.
Go to System Preferences => Keyboard and set Full Keyboard Access to All Controls
Another useful tip is holding down the Shift and Option keys while changing the volume or brightness on your Mac, to change them in quarter increments.
There are a bunch more, and it's an awesome read and worth saving in your favorite bookmarking service. While you're at it, add the Apple area on Stack Exchange to your bookmarks while you're at it.