How can I make Safari show the URL when I hover over a link?

There are two options. One is a setting, and the other is a workaround.

Here are instructions on how to toggle the setting using the Menu Bar:

1. Go into the View menu.
2. Select "Show status bar".
3. Now, there will be a URL that pops up when you hover over a link, and also tells you if it's going to be in a new tab:

You can also press ⌘ + / to toggle this setting at ease.

The second option is a workaround for when you drag links. Here's how to get this feature to appear:

1. Click and drag the link out of where it will originally appear.
2. At your mouse will be a small box telling you the title of the new window and its URL, although it may be shortened.
3. You can also drag the link to the plus sign in your tab list, out of the window, or in a folder/the desktop to open that URL in a new tab, new window or save it to your computer, respectively.

I believe this Safari Extension did precisely what you are looking to do, but it's now abandoned: Ultimate Status Bar.