Introduction to Pushdown Automata: A lecture describing the limitations of FSA, introducing PDA and context free grammars.
Non-determinsitic Pushdown Automata: A definition of NPDA with examples and a proof that NPDA accept context free languages.
Push-Down Automata: Notes for a linguistics course defining PDA and showing their relation to context free languages.
Pushdown Automata: Definitions and examples of deterministic and nondeterministic PDA and an example to show that the family of deterministic PDA is a proper subset of the family of nondeterministic PDA.
Pushdown Automata: A site with a formal definition of PDA, some examples and a proof that PDA accept context free languages.
Pushdown Automata and Context Free Languages: An article with basic definitions and a proof that the languages accepted by PDA are context free languages.
Pushdown automaton: A Wikipedia article with a definition and simple example.