and what they're missing
see also http://en.wikipedia.org/wiki/Turing_machine_equivalents , http://en.wikipedia.org/wiki/Model_of_computation , http://en.wikipedia.org/wiki/Category:Models_of_computation , http://en.wikipedia.org/wiki/Abstract_machine , http://en.wikipedia.org/wiki/Turing_completeness , http://en.wikipedia.org/wiki/Church%27s_thesis, Gandy and Sieg's notions of informal computability ( http://en.wikipedia.org/wiki/Church%27s_thesis#Later_developments ), http://en.wikipedia.org/wiki/Hypercomputation