In a book by John Gribbin titled Cosmic Coincidences, he has some statistics within the first few chapters on the probability not only of earth-like planets but also of carbon-based life. I no longer have my copy of the book, but the numbers are a lot higher than what I would have expected. Now whether or not they are close enough to earth for close encounters or even technological as a species is another question. I'll have to see if the library has a copy and get those numbers again.
It's like walking down an empty street, listening to your own footsteps. But all you have to do is knock on any door and say, "If you'll let me in, I'll live the way you want me to live. And I'll think the way you want me to think." And all the blinds will go up, and all the doors will open, and you'll never feel lonely. Ever again.
~Henry Drummond, "Inherit the Wind" (1960)