In her NY Times blog, Olivia Judson gives an ovation to Archaea.  This domain of organisms gets none of the recognition of their more famous kin, Bacteria and Eukaryotes despite their unusual biology.  As she describes, some members of the group Archaea have extreme tolerance to temperature and pH, thriving in the boiling acids found in hot springs and the bottom of ocean vents.  All known methane producing microbes are found among the Archaea.