Scientists at New Zealand's University of Auckland have estimated the number of Earth-like planet's in our galaxy alone to be around 100 billion, which means that their could be 1 Earth-like planet for ever 2 stars in the Milky Way (around 200 billion stars in total).
There could be 100 billion Earth-like planets say astronomers - Telegraph
They have based their estimates on observations by the Kepler Space Observatory. More than 800 exoplanets have been discovered so far, mainly from observing dips in the light of the star as the planets pass in front of them or from wobbles in the stars position (due to the gravitational influence of orbiting planets). This number is rising rapidly, and as more observatories are planned to be launched in coming years (such as the James Webb Space Telescope), the likelihood of observing smaller planets (which may be Earth-like in size) will increase as the sensitivity improves.
Considering the sheer number of planets like our own that could potentially be found within our own galaxy, I think it would be exceedingly unlikely that ours would be the only one to harbour intelligent life, let alone life itself. Your thoughts?