Recursively Enumerable Language: A Wikipedia article which defines Turing machines, gives examples and describes equivalent models and the universal Turing machine.
Turing Machine Languages: This article shows the relation between recursively enumerable languages and Turing machines.
Turing Machines: Article in Stanford Encyclopedia of Philosophy.
Turing Machines: Examples of Turing machines, the Church-Turing thesis and a discussion of uncomputability.
Turing Machines and Computability: A discussion of decision problems, Turing machines, noncomputable problems, Goedel numbering and the halting problem.
Virtual Turing Machine 2.02: An online Turing machine for which the reader can enter the input, start state and transitions and see how it operates.