an abstract machine devised by Alan Turing to model computation Wikipedia entry → http://www.wikipedia.com/wiki/Turing_machine

• Universal Turing Machine
• Deterministic Turing Machine
• Probabilistic Turing Machine
• Non Determinisitic Turing Machine?
• Limit Turing Machine
• Topological Turing Machine

a 272 bit universal computer is described here → http://www.cwi.nl/~tromp/cl/cl.html