Cheat mode:
In the main menu, push Circle, Left, Right, Circle, Up, R2. A CHEAT choice will become visible on the options prompt, allowing God mode, every weapon and unlimited ammunition, unlimited oxygen, no chestbursting, and level choice.

Research mode:
In the main menu, push Square, Up, Down, Circle, Left, R1. A RESEARCH MODE choice will become visible on the options prompt. This mode allows you to change some of the game's Aliens and Facehuggers (shrinking them, making them wire-framed, semi-transparent, etc.). This is done by highlighting various grid squares as follows.

1: Small aliens
2: Quarter-size aliens
3: Tiny aliens
4: Half-size aliens
5: See-through green aliens
6: See-through red aliens
7: Invincible aliens
8: Tiny face-huggers
9: Large eggs
10: Hard enemies
11: Very hard aliens
12: Faster aliens
13: Red eggs and face-huggers
14: Slow aliens
15: Faster, weaker aliens
16: Faster, weaker face-huggers

Combinations of these grid squares can give different Alien effects. There are other combinations for large, small, and variable colors. The eggs and face huggers do not always show up immediately, however pushing Start brings up the effects.

6 + 13: Clear red aliens, eggs, and face huggers (normal size)
6 + 7 + 12 + 13: Clear yellow aliens, eggs, and huggers, and greenish-yellow guardian aliens (normal size)
5 + 7 + 12 + 14: Clear teal aliens, eggs, and huggers, and bluish guard aliens (normal size)
5 + 6 + 7 + 12 + 13 + 14: Clear green aliens, eggs, and huggers (normal size)
All tiles: Large green everything