1. Extensions and Restrictions of Turing Machines

2. Extensions: (see this link too)
Many extensions of Turing machines are no more powerful than the originals
This is evidence that Turing machines are a universal model of computation
Extensions: (see this link too)
Multiple tapes
Multiple read write heads
Random access
Nondeterminism
Two or more dimensions

3. Restricted versions of Turing Machines
Infinite versus semi-infinite tape
Restricted versions of Turing Machines
Only move to the right
Only one state
Never change a tape symbol
All these restrictions make Turing machines weaker