Olympus Mons is located on Mars and is the largest volcano (and mountain) in the solar system. It is in the Tharis region, one of Mars' two main volcanic areas, the other being Elysium. At 27km, the height of this shield volcano is astounding, Everest is 8.848km. However it is the width (550km) that is truly mind blowing, to put this into context it is the size of Arizona or if you were standing on Mars you wouldn't be able to see the summit of Olympus Mons (and not just because you'd probably be dead). The caldera (central volcanic crater) is a humongous 66 by 83km and 3km deep.
Mars unlike Earth is lacking in plate tectonics which is why it is believed that Olympus Mons has grown to such a large size. With each subsequent volcanic eruption more lava is added to this fixed hotspot. It is relatively difficult to measure the volcanism of Olympus Mons due to its extraterrestrial nature. However lava flows have been seen as recently as 2004 and due its relatively young age, it is expected that there will be more volcanic activity.
This website offers more information on Olympus Mons and has some excellent pictures.
OlympusMons.com - Your Guide to Olympus Mons - the largest volcano in our solar system.
Thanks to Vanna 'volcanoes in space' Barber for a heads up on this volcano and a happy summer solstice.