GTA IV - Glitch at hospital

Go to the hospital in NorthHolland, drive a SUV or other large car up to the red shelter at the front door. Climb onto the car, and jump from the car to the shelter. Walk to the back of the shelter and down onto the ledge on the wall where the hospital cymbal is, and then climb back up onto the red shelter, Niko will start to fall through the shelter and then jump several times before standing still.

Crackdown - Drop cop cars

If you pick up a cop car and jump into the air and drop it, it will fly a little ways and land on the ground. Although this will sometimes kill the cops in the car, they won't attack you.

Dead Rising - Escaping crowds of zombies

Once unlocked, you can use the jump kick to knock several zombies out of your way.

Dead Rising - Katana and Battle Axe

In the upstairs part of the entrance plaza, there is a store that has both katanas and battle axes. This is the closest store to the security room that has these weapons.

Crackdown - Cars go crazy

Sometimes if you shoot all the tires off of a car, it will start sliding around out of control.

Dead Rising - Using the sledgehammer

If you get a sledgehammer (from the warehouse or from a hardware store) you may not realize it has two attacks. The first attack is a slamming the hammer down towards an enemy, sometimes beheading them which works great for a single opponent, but you can also hold x down to swing the hammer around, hitting several enemies at once with less damage.

Turok - Using the knife

If you think the knife is useless against heavily armed enemies, think again. It's possible to clear out an entire room of human enemies with just the knife, and even easier against dinosaurs.

GTA IV - Cowardly Helicopters

If you shoot both gunners out of a helicopter leaving only the pilot, he will fly away.

Crazy Rockets in GTA IV

If you shoot a RPG at the ground, a lot of times it will slide across the ground, fly up in the air, or even turn back towards you.

Gta San Andreas - Invincible Vehicles

This trick requires a cheat.

If you use the Vehicle of Death cheat (makes all vehicles destroy other cars like a tank) it also makes the vehicle you're in invincible, including the Hydra, that means you can run it into a building and it won't take damage, not that you'd want to do that, but it's still a good example.