On a modern Linux system, the middle mouse button does four incredibly useful things:
It performs fast pasting. Highlight something in one window, then click the middle mouse button in another window (or another part of the same window) and the highlighted text or graphics gets pasted in. Copy'n'paste only takes a few seconds more, but I use this so often that I'm sure the seconds would add up to days.
It goes to a URL or performs a search in a web browser. Highlight a URL anywhere and then middle-click in a blank part of your Firefox (or Konqueror, or Galeon, or Epiphany) browser window and you'll go to that page. Or highlight a word or phrase without punctuation and click the middle mouse button in Firefox to perform an "I'm feeling lucky" Google search.
It opens a link in a new tab in Firefox, leaving your original page open.
It scrolls to a particular spot in your scrollbar area. If you want to scroll halfway through a document. put your cursor halfway down the scroll area and middle click -- the scrollbar will jump to where your cursor is located.
X11 conventions power some of these features, toolkits or applications power others, and some of these can be configured on other operating systems. But they're there by default on Linux, and I love the way they save me time.
No middle mouse button? Press in your scroll wheel or click the left+right buttons at the same time.