Holding down the Ctrl button* while clicking on a link will open a webpage in a new tab or window. For those stuck using Internet Explorer (if you're using it by choice, see: We Don't Support IE), the Ctrl + click method also bypasses the pop-up blocker, for when a poorly designed website is forcing content to open in a new Window, which IE often perceives as a pop-up. Opening links inside Microsoft Word and/or Outlook also require a user to hold down Ctrl while clicking.
Bonus: If you are a firefox user who likes to open everything in a new tab, check out the Tree Style Tabs add-on for organizing your tabs into collapsible hierachies; tabs can also be displayed vertically allowing more to be visible at once.
*Rumor has it that the CMD button does the same on a Mac.