Pizza Pie For You And I

Whilst this trick won’t apply for real life, as sad as that may be, you can scam the pizza delivery guy out of a free pizza in The Sims 2. Call for a pizza, but leave the guy hanging around at the door. Eventually, he’ll give up; but sometimes he’ll leave the pizza on the front porch- where you can go and collect it, free of charge.
Free Bird!

Every rock fan worth their salt knows hecklers love to shout “Free Bird!” at concerts, referring to the famous Lynyrd Skynyrd track. The said track actually appears in Guitar Hero II, even though the game says that “they don’t really want you to play Free Bird, they’re just heckling you”.
The song also appears as a master track (i.e not a re-recording by the in house rock bands) in Guitar Hero: Smash Hits.
R-R-R-Reload

There is a quicker way to reload in Halo. Well, technically. The trick involves tapping X twice quickly, then switching weapons with Y. Whilst this isn’t really a reload (the gun reloads ‘in the background’), this trick means you can use another weapon whilst it’s being reloaded.
Tomatoes Are Good For You

The Maxim Tomato (Kirby’s favourite food) is a healing item in both the Kirby series and in the Super Smash Bros. series. Both items heal a large amount of health in both games- 50% of damage in Super Smash Bros (100% in the N64 edition) and fully restoring Kirby’s health in all Kirby games it appears in.
Funnily, the Maxim Tomato actually absorbs damage in Super Smash Bros, if someone attacks you whilst you’re being healed.
I Could Eat an Octorok!

Scattered around the seas of The Legend of Zelda: The Wind Waker are six Big Octoroks. Defeating them all will get you two Pieces of Heart, 300 rupees, and a Magic Meter upgrade.
Sleeping on the Job

I mentioned in a previous post that Sonic will jump off the screen if left inactive for three minutes in Sonic CD. However, hackers of Sonic the Hedgehog 3 discovered that Sonic is supposed to fall asleep with boredom, but for whatever reason, he doesn’t:

In the final game, Sonic clicks his fingers and points in the direction he’s facing.
Pac-Man Fever

Pac-Man, for such an ancient game, is far more complex in AI than most people think. Those ghosts aren’t just stupidly chasing you.
Just to delve into it slightly- the red ghost (Blinky) is actually programmed to move faster when a set number of dots are left - 20 from the first level, moving up by ten until reaching maximum speed as soon as the level loads.
Wonky Wario

Wario, the ‘Bizarro’ version of Mario, used to look a lot derpier at first. In Super Mario Land 2: 6 Golden Coins, Wario was crosseyed and was far bulkier; though we can safely say that he’s clearly lost some weight and had laser eye surgery since.
Soar To The Heavens

Want to give Mario a proper send off when he dies? In Super Mario 64, if Mario falls into a cannon with no health left, he can still be fired from the cannon, only to die when he hits the floor.
Interestingly, if Mario has a Flight Cap on, he can fly around as normal. Hey, if you’re able to fly, best take advantage of it and pick up some stars, huh?
Kirbtroid

In Kirby Superstar Ultra, gain the Stone ability by swallowing a Rocky or using a Copy Statue in Milkyway Wishes. Keep hammering away at B, and Kirby will eventually turn into either Samus from Metroid, or a golden Mario statue.