A Wikipedia article which defines Turing machines, gives examples and describes equivalent models and the universal Turing machine.

A simulator which runs included programs such as a palindrome detector and also allows writing of programs.

A discussion of decision problems, Turing machines, noncomputable problems, Goedel numbering and the halting problem.

An implementation of Turing Machine in C++ prepared for the Wroclaw University of Technology for the Mathematical Complexity of Algorithms course.

Article in Stanford Encyclopedia of Philosophy.

Basic introduction Turing machines, the Church-Turing thesis and a discussion of computability. Part of the course taught at the University of Hawaii at Hilo.