Half-Life 2

Press Left(4), Black, Right(4), White during game play. Return to the main menu and start a new game to have all chapters unlocked.

Anytime during gameplay enter:

After Dog attacks the dropship and and it drives off, go to the bent down fence and jump over it. Walk down a few steps and on your right is an open door. Go in it and to your left you will see a stairwell blocked by a couch and debris that cannot be moved. Crouch jump over the couch and go up the stairwell. You might want to bring a box with you so that you can put it on the ground in front of you then jump on top of it and make longer jumps.

Later in the game when you are fighting alongside Alyx, at one point you will encounter a door where she tells you to go on without her, while she heads up to the communications room to unlock doors for you. Push her into the corner left of the door, then run in and watch it close behind you. As the next area is loading, she appears next to you. This clone does not talk, and is not always there. However, later when both of you fight a group of soldiers, there are two of them.

Immediately after the Combine outpost with the crane, you will jump a gap in the buggy and get attacked by a gunship. There is a crate full of rockets in the van in the middle of the road you can use to shoot it down. Clear the road with the gravity gun and drive the buggy through. After passing two more telephone poles, there is an opening on the left with another broken-down van and a rock structure on top of the hill that vaguely resembles Stonehenge. This van has three crates with medkits inside if you need them. Drive the buggy past the van and straight at the rock formation. It should climb the wall and allow you to walk to (or off) the edge of the world. Note: You will die if you walk off. Also, there are some invisible walls as soon as you get up there, but you can travel a number of miles along the ridge (but no farther along).