I like having three buttons on my mouse. I use the third button all the time! Do you? If you use a Windows PC like me, then you too have three buttons... (unless... how OLD is your computer?) The third button is also known as the "wheel" on your wheel mouse. But it is more than a wheel. Did you know you can click the wheel down as a third button?
My favorite use of the middle button (i.e. clicking down on the wheel) is to open a link on a webpage in a new tab. What's cool about having it open a new tab is that you can go to the article and then easily come back to the page that launched you there. Sure you could use the back button, but if you're like me, one article takes you to another and another and you are several steps away from a single back step by the time you are done.
Two sites where I often am clicking away with my middle button are ActiveRain and Google. So you start your morning on the ActiveRain home page? How many of those featured articles do you read? I start at the home page and then "click, click, click, click, click" along with my middle button on the articles there to create a reading list. Now I can start reading the first article while the other pages are loading in the background and when I'm ready to switch, they are instantly ready having been already loaded for me (no waiting!)
Google is the other place where my middle button gets used frequently. Your Google search turns up a long list of results. On a more difficult topic, which article is going to be the one you want? It's going to take several click-throughs to find it. I click the page links with my middle button so I can easily get back to my search results list.
Just as a side note, alternatively, you can Ctrl+Click a link (clicking with the regular left button on the mouse) to also open a link in a new tab. I have a hunch Macs do command+click or something like that (and mac readers, please tell us if that is so)