Introduction to Petrov's Defence (also known as Petroff's Defence and Russian Game):
Wikipedia about Petrov's Defence
1.e4 e5 2.Nf3 Nf6 3.Nxe5 Nxe4
Games Petrov at Chessgames.com
Games after 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.Nc3
Games after 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.d4 d5 6.Bd3
Games after 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nxf7 Kxf7
Games after 1.e4 e5 2.Nf3 Nf6 3.d4 exd4 4.e5
Games after 1.e4 e5 2.Nf3 Nf6 3.d4 Nxe4 4.Bd3 Nc6
Games after 1.e4 e5 2.Nf3 Nf6 3.d4 Nxe4 4.Bd3 d5 5.Nxe5
All games at Chessgames.com after 1.e4 e5 2.Nf3 Nf6