Grand Theft Auto

Take control of a train:
Get on a train at any train station and press Ctrl to take control of it. Drive it with the cursor keys.

Bomb Shop:
Drive a car (a small one is easiest) into a garage with an X over the door to have a bomb fitted. You need at least $5,000 for this. Stop the car somewhere, press Fire to arm the bomb and get out within about five seconds.

Easy tank (Liberty City, Heist All Mighty):
Run someone over and wait for the ambulance. Steal the ambulance (a police car also works) and drive into the army base, where you can take the tank. It only works with an ambulance or a police car.

Monster Bug (San Andreas, Mandarin Mayhem):
In Central Richman a Monster Bug is parked in one of the driveways. Get in it for a two-minute Kill Frenzy.

Gang followers:
Walk up to a group of Gurangas (the noisy groups that run around after each other) and beat up the one in front instead of shooting him. After enough hits the group starts following you almost anywhere, which is a good way to get a Guranga with a machine gun or flamethrower on your side.