Gradius Collection Cheats

Pause in any game under their easier difficulty setting, then press Up, Up, Down, Down, Left, Right, Left, Right, L, R. All weapons and power-ups will be unlocked.
In the Japanese version of the game, press Up, Up, Down, Down, Left, Right, Left, Right, X, Circle.

This is not necessarily an awesome cheat, but it is something that I happened to stumble upon while playing. Thus far, this code has worked for Gradius II so I imagine that it will work for all of the games.

Basically, get a few power-ups, and then save your game. After saving, load your game and you will notice that the power-ups that you earned are still there. So, if you keep doing this, you will eventually have a full arsenal.