Also note that over time expectations and practices change. What is true today may not be true tomorrow. It is important to be flexible and meet the needs of our current users.
There are many cases for opening a link in a new window. There are many cases for not opening in a new window.
Here are some guidelines and any exceptions need to be carefully considered whether or not they are worthy exceptions:
Navigation links should not open in a new window. Top navigation, side navigation, footer navigation links should not open in a new window. Why? Because the user is signaling that they are done with the page and wish to move on. If they wish to come back they will use the back button.
One type of exception: Help, chat, or Ask a Librarian.
Links in the body of text have more leeway. Foot notes, reference, citations, tangents, and supporting documents are all acceptable reasons to consider a new window. Link pages, or pages that serve up a list of jumping off points such as databases or services, are not examples of tangents even if the user is leaving your site. If they want to come back they will come back.
Non-web native documents. PDFs, Word Documents, etc, sometimes open in the browser but with no back button. These should open in a new window.