Cool article calculating the odds of alien life:
The odds given are a little generous, though. For example, he estimates the odds of a civilization evolving on planets with intelligent life at 50%. I think that is way high. The evolution of intelligent life depends on a lot of unique evolutionary events:
My own speculation on this issue is that it is almost certain that bacteria (or some simple form of life) exists somewhere in the universe, and that complex life (things like plants and animals) probably exist elsewhere in the universe, and that it is possible that a few other civilizations exist.