Europa, Moon of Jupiter

Europa, moon of Jupiter, is strongly evidenced as having subsurface oceans of liquid water, making Europa a very strong, if not the strongest, non-Earth candidate for harboring life.
Tidal flexing forces are believed to provide the heat that keeps such subsurface oceans liquid. The tidal flexing is caused by gravity between Europa and Jupiter, and variations in the gravitational force on Europa due to variations in Europa’s distance from Jupiter at different points in its elliptical orbit.
Possible hydrothermal vents in the subsurface oceans of Europa (which could be caused by tidal flexing) could hypothetically provide a habitat for life, just as life clusters around hydrothermal vents in the oceans of Earth.
Resources include: http://en.wikipedia.org/wiki/Europa_(moon)

Fosdick's Astrobiology