Metal Gear Solid 4: Guns of the Patriots

When entering the password into Otacon's terminal, input 14983. Although incorrect, Snake will (invisibly) gain 100K of Drebin points. Run through the game a few more times with this and the no kills and no alerts bonuses, and you should amass quite a sum of Drebin points.

After dying, call Rose to get a conversation about Snake having dreams of being killed or a comment from Snake saying that he feels like he's died once before.

There are two posters of Akina Minami in the Vista mansion. Look at them in first person view and Snake will say her name and his Psyche gauge will increase.

Equip the Altair costume after unlocking it. There are some changes Snake will make to his posture. First, he stands with his hands by his sides and outstretched just like Altair, as opposed to hanging down in front of him with his fists clenched. This change will only occur when you do not have a weapon equipped. Second, where Altair had the ring finger on his left hand missing, Snake bends his up to give a similar visual effect. He will even keep his finger bent during intermission sequences in which the costume you are currently wearing appears.

If you fight Mantis with a Sixaxis controller, the ending is different than if you were using a DualShock 3 controller.