You can quickly add items to the Finder Dock or Sidebar with this easy keyboard trick. Although Apple probably meant for it to be primarily used for folders, it actually works on a variety of different file types.
If you want to add a folder (or file) to your Finder Dock simply select it and press Shift+Command+T and Mac OS X will add it to the Dock. If you want to add it to the Finder Sidebar, press Command+T and Mac OS X will add it to the Finder Sidebar.
Take note that you can also do either of these tasks from the Finder File menu using the shift key to make Add to Dock appear in that menu, which normally displays Add to Sidebar, while holding the shift key.
In the example image above, I selected my Articles Archive (MBA) folder by clicking it and then I pressed Command+T. The folder was then added to the Sidebar for quick and easy access to the archive of articles I’ve written over the last month or so. Because I use it so often, it is nice to have it readily available in every Finder window I open.