Mac: In old versions of OS X, you could simply drag any icon you wanted into the toolbar to add it. You can still do that, but as Macworld points out, you'll need to hold down the Command key in modern versions of OS X for it work. If you use a particular app a lot, or there's one folder you always want access to in Finder, the toolbar is a good place for a shortcut. Just find the file, app or folder you want; tap and hold the Command key; then click and drag the icon to Finder's toolbar. Now you can access it right away. This is probably most useful for something like an Automator workflow, but it's really up to you how you use it.