I think most people would be surprised to find out that the most volcanically active body in the solar system is actually one of Jupiter's moons, Io.  It has more than 400 volcanoes, and the amount of geologic activity under the surface of the moon is astonishing.  Apparently it is the result of the friction and gravitational tug of war between the massive planet of Jupiter and other moons nearby.

Some of the volcanoes are taller than Earth's Mount Everest, and sulfur plumes that extend hundreds of miles into space are common. The large amount of activity also creates a very powerful magnetosphere and long lava flows that make up most of Io's surface geology.