Automata Theory - Wikipedia article on finite state machines with links to articles about pushdown automata, linear bounded automata and Turing machines.
Theory of Automata and Formal Languages - Survey of finite state automata, pushdown automata, Turing machines and the languages accepted by these devices. [PDF]
Turing Machines - A brief survey of finite state automata, pushdown automata, linear bounded automata and Turing machines.
Types of Automata - Intuitive definitions of automata from the more complex Turing machines to the simpler finite state automata. [PDF]