Arsenic-Eating Bacteria

In addition to many other types of extremophiles―organisms that have evolved to live in conditions where most Earth life forms can't survive―we now have arsenic-based bacteria:

http://news.discovery.com/earth/arsenic-bacteria-alien-life.html

Taken from the already extreme environment of Mono Lake in California, these bacteria were forced under lab conditions to substitute arsenic for phosphorus in their cellular chemistry. The discovery of organisms that can do this has exciting implications for the search for extraterrestrial life, as the media have pointed out. "Life as we know it" covers a much broader range than our oxygen-breathing kin or even our more distant carbon-dioxide-absorbing plant cousins.

From Wikipedia, here's a list of other categories of organisms thriving in extreme conditions:

Acidophile
An organism with optimal growth at pH levels of 3 or below
Alkaliphile
An organism with optimal growth at pH levels of 9 or above
Cryptoendolith
An organism that lives in microscopic spaces within rocks, such as pores between aggregate grains; these may also be called Endolith, a term that also includes organisms populating fissures, aquifers, and faults filled with groundwater in the deep subsurface.
Halophile
An organism requiring at least 0.2M concentrations of salt (NaCl) for growth
Hyperthermophile
An organism that can thrive at temperatures between 80–122 °C, such as those found in hydrothermal systems
Hypolith
An organism that lives underneath rocks in cold deserts
Lithoautotroph
An organism (usually bacteria) whose sole source of carbon is carbon dioxide and exergonic inorganic oxidation (chemolithotrophs) such as Nitrosomonas europaeaNitrosomonas europaea ; these organisms are capable of deriving energy from reduced mineral compounds like pyrites, and are active in geochemical cycling and the weathering of parent bedrock to form soil
Metallotolerant
capable of tolerating high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc; examples include Ferroplasma and Cupriavidus metalliduransCupriavidus metallidurans
Oligotroph
An organism capable of growth in nutritionally limited environments
Osmophile
An organism capable of growth in environments with a high sugar concentration
Piezophile
An organism that lives optimally at high hydrostatic pressure; common in the deep terrestrial subsurface, as well as in oceanic trenches
Polyextremophile
An organism that qualifies as an extremophile under more than one category
Psychrophile/Cryophile
An organism capable of survival, growth or reproduction at temperatures of -15 °C or lower for extended periods; common in cold soils, permafrost, polar ice, cold ocean water, and in or under alpine snowpack
Radioresistant
Organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation, but also including organisms capable of resisting nuclear radiation
Thermophile
An organism that can thrive at temperatures between 60–80° C
Thermoacidophile
Combination of thermophile and acidophile that prefer temperatures of 70-80 C and pH between 2 and 3
Xerophile
An organism that can grow in extremely dry, desiccating conditions; this type is exemplified by the soil microbes of the Atacama Desert

With so many kinds of life to choose from even on our own planet, fictional extraterrestrials need not be restricted to "rubber forehead aliens," human in all but cosmetic features. As a character in Heinlein's HAVE SPACE SUIT, WILL TRAVEL mentions, spiders don't look anything like us, yet they live in our houses. Suppose creatures who've evolved in totally different environments have enough intelligence for us to interact with? For Terran-alien love and mating, we need to meet other humanoids, but we can also hope for allies and friends of wildly different species. To invoke the Vulcan ideal: "Infinite diversity in infinite combinations."

Margaret L. Carter
Carter's Crypt