Mount Everest gets all the credit, but Hawaii's Mauna Kea (an idle volcano that's been around for, you know, a million years) is technically the world's tallest mountain. Technically. While it stands 13,800 ft above sea level, more than two thirds of the hill is actually submerged underwater, which brings the total height to 33,136 ft.
Best part of that mathematical sleight of hand is that you can get to the top of Mauna Kea without training, oxygen masks, or even having to ask a dude to carry your fanny pack. All you have to do is sign up for this tour and hop on a mini-coach.