Metal Gear Solid Easter Egg - Psycho Mantis Reads Your Mind!

When you meet Psycho Mantis, the game looks through your memory card for saves from other Konami games. Mantis will then make comments referencing those games. For instance, if you have a ISS Soccer saved game, then he'll mention that you seem to like playing soccer, and so on.

The following game saves (and probably some others) provoke responses:
Castlevania: Symphony Of The Night
Policenauts
Silent Hill
ISS Soccer
Suikoden

This also occurs in later versions and sequels, including the GameCube version.