Gliese 581g (or GJ 581g) is the third planet in the Gliese 581 system. It is a potentially habitable planet 20 ly away in the constellation Libra. Its sun is a small red dwarf star, so the habitable zone of the system is much closer to its sun than ours. So close that the stars gravity stopped g's rotation! This is called tidal locking, where one side of a world is always facing the thing it orbits.
G's 2.5 times bigger than Earth and has water on its day side (and ice on its night side), so it is fit for human colonization.