- Kingdom Crenarchaeota
- Kingdom Euryarchaeota
- Kingdom Korarchaeota
As mentioned in archea, this is the third branch of life. Originally, there was the five kingdom system (and before that, all sorts of stuff) which tended to concentrate on macroscopic life. The three empire system (NB: most people call them Domains, I just like Empire) was concieved mainly because of the discovery of Archaeans. Molecular phylogenetic techniques allow more detailed analysis of the relationships between species; and from this has come some understanding of the ancient split between the two branches of prokaryotes - Bacteria and Archaea. They share some of the fundamental components (notably the replication and translation machinery, and histone like proteins) with eukaryotes, and some with thier fellow Bacteria - although it is difficult to distinguish origins at the tangled base of the tree of life.
Also known as extremophiles (because many are, although eukaryotes can live in extreme environments too) these prokaryotes live in all sorts of weird and wonderful places. The first hyperthermophiles were discovered in Hydrothermal Vents on the ocean floor, and are still being discovered in ice, rock and soil. The bubbling sulphorous waters of the Yellowstone Spring also yielded surprises, since the temperature and acidity are too high to support other types of life.
However, on a cautionary note, it is possible to get too carried away about these beasties. Some have suggested that there are 'superthermophiles' that live even closer to the core of the earth than deep subsurface organisms. Gould's deep hot biosphere is exactly this kind of (wild?) speculation about vast microbial communities under the surface. Even more unlikely are theories of exobiology (or astrobiology, or whatever) that claim various planets or moons (Mars, Galena) that might be inhabited by extremophiles. This seems extremely unlikely. | 
