Extremophiles - Organisms that thrive in what, for most terrestrial life-forms, are intolerably hostile environments. Example: WATERBEARS! They can live almost anywhere and can survive in all sorts of extremely hostile environments, even outer space, (at least for a little while)!
Thermophiles - Thermophiles are microorganisms that live and grow in extremely hot environments that would kill most other microorganisms. Example: Thermus thermophilus, which can survive in the Hot Springs at Yellowstone.
Acidophiles - Organisms that thrive under extremely acidic conditions. Example: Found in Lechuguilla Cave, Carlsbad, New Mexico, where the pH is 0.0, which one website claims is about as acidic as battery acid!
Halophiles - Organisms that live in environments with very high concentrations of salt. Example:
Halobacterium salinarum can exist in the Dead Sea.