The odds of finding a habitable planet depend on how you define "habitable." All stars are surrounded by temperature zones where water can exist as a liquid. Rocky (as opposed to gas-giant) exoplanets in these zones are probably underrepresented in the current count, since they are small and their observable effects are near or below the detection limit of current search techniques.

http://en.wikipedia.org/wiki/Extrasolar_planet#Detection_methods

Therefore, there are probably numerous planets (or moons of gas-giant planets) in the galaxy capable of holding liquid water.

Life on Earth originated in an atmosphere without free oxygen, consisting mainly of carbon dioxide, water vapor, and nitrogen:

http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit5/atmos.html

Such a planet could support life, but not human life. If the presence of free oxygen is considered a requirement for a "habitable" planet, then all habitable planets already have native organisms capable of photosynthesis (oxygen is too reactive to remain free in the atmosphere without constantly being replaced). The probability of such simple alien life is impossible to calculate with any accuracy; however, life on Earth first appeared shortly after a solid crust had formed:

http://www.news.harvard.edu/gazette/2004/10.07/15-origins.html
http://en.wikipedia.org/wiki/Timeline_of_evolution#Hadean_eon

This suggests that under suitable conditions, life may develop fairly easily, in which case life (at least primitive life) would not be uncommon. However, given that it took more than 2 billion years for life on Earth to evolve to the multicellular level, it is possible that more advanced forms of life may be quite rare.

BTW, according to the latest data, Gliese 581c is probably too hot to be habitable, but Gliese 581d is now thought to orbit within the habitable zone.

http://www.centauri-dreams.org/?p=1228
http://en.wikipedia.org/wiki/Gliese_581

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