http://en.wikipedia.org/wiki/Gliese_581_c
Scientists just announced the discovery of an earth-like planet within the star's habitable zone only 20.4 light years away from earth. The specualtion is that so far it has all the conditions for liquid water on its surface. Liquid H2O, of course, is useful if not essential for an innumerable number of chemical reactions including several involving limestone and cave formation.