Gradius

CheatingDome.com

30 lives, player one

At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.

30 lives, player two

At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Select, Start.

Continue playing

Quickly press Down, Up, B, A, B, A, B, A, B, A after you've lost your last ship.

Full power-up

While playing, pause then press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.