In Tetris, you occasionally will have to maneuver pieces so that they fit in spaces where it doesn't initially look like they're going to fit.
Like this T-piece, for example.

It would totally fit in that hole on the left column if some of those pieces weren't in the way.

But if you let it get partway in

And then rotate it at the last second, it slides into place
Which is easier to show in animated .gif form

This is a pretty useful maneuver, since it lets you slip those T-pieces in places where they wouldn't normally go.
The first NBA Jam game had a few hidden characters, but it has nothing on the Tournament Edition of the game, which went way over the top.
For instance: if you enter your initials as GRM

And your birthday as OCT 31


Your player turns into the Grim Reaper! Who looks kinda funny rather than scary.

Just be aware that this doesn't work on all revisions of the game, just v.1 and v.2.
Playing Super Puzzle Fighter and tired of playing as Dan (or one of the regular characters)?
Well, then I've got a present for you!
Start the game as Player 1

Then, hold down the Start button and press Down, Left, Down, Left, Down, Left, Down, and then press and hold Left on the Joystick (while still holding Start, remember). While holding all these down, press one of the Action Buttons to select Morrigan. Done right, she'll turn into Akuma!

Who cuts a much more imposing profile.

And should make the game a little more fun.
Occasionally, while you're slinging Root Beers around in Tapper, you'll notice that occasionally a patron will drop a tip on the counter.

If you grab it, you get a 1500 point bonus, and some 'entertainment' plays

But I typically avoid picking them up for 3 reasons:
This doesn't mean that you should necessarily avoid grabbing all of them, but I'd avoid them until you're ready to increase the game's difficulty.
If you play Bubble Bobble enough, you might begin to suspect that the collectible items that appear are essentially random.
But you'd be wrong.
Some of the more useful items are the bubbles that hold the letters that spell the word EXTEND. To get them, all you have to do is encase a lot of enemies in bubbles and maneuver them so that they all pop at the same time.

If you popped enough, when the next stage starts you'll notice that the lettered bubbles start pouring in!

In fact, you get two less than the number of enemies that you popped at once (pop 3 enemies, get 1 bubble; pop 5, get 3 bubbles; etc.)
And this works on every version of Bubble Bobble that I tried it on.
Whenever Dan appears in a Street Fighter game, you can bet that he's going to be way underpowered and tough to use, which makes him quite the challenge to get good with.
Take his "ultimate" maneuver, for example. So if you're playing as him and you get the Hyper Combo gauge up to maximum (L. 3) and you press:
Hard Punch, Light Kick, Back, Light Punch, Light Punch
He moves in for the kill!

Except that when he's done, his opponent loses very little energy, and Dan loses almost all of his!

So why would you ever want to do it?
My best guess is that you'd use it to show off as you're beating someone senseless with Dan. But, since he's kind of hard to master, I've never actually been able to prove that.
In The Punisher, you find all kinds of things to pick up, some, like moneybags, food, and precious gems, are good for some points, but others, like pipes, shurikens, axes, chairs

oil drums

And all kinds of other things can be used as weapons.
But there's another reason to pick them up.
At the end of each stage, you get bonus points tallied up, and there's a special category for item pickups.

You get some bonus points for everything you've picked up in the level, whether you smashed it over some malefactor's skull or not. So you should definitely spend a few extra seconds picking up everything you come across in order to build up an impressive score.
Unless having your initials across the top of the game screen doesn't mean that much to you.
You have two ways to clear levels in Pengo, you can either make all three of the blocks with diamonds on them touch each other or you can use the regular blocks to squish the blob-things that fill up the levels.

But, what might not be immediately obvious is that you can use the boundaries of the stage to your advantage.
Just walk up to the edge and press the Push button. You'll cause shockwaves to go down the length of it, stunning any enemies that are near it.

Giving you a few precious seconds to formulate a counter-strategy.
Today, I'm going to share one a quick-'n'-easy two hit combo with Street Fighter's Guile.
First, you're going to want to hold Back on the controller to charge for the Sonic Boom. Then press Forward and Weak punch to throw it.

Keep holding Forward to walk toward your opponent, trailing your projectile, after it hits,

follow it up with a Fierce Punch.

Done right, you'll be able to hit your opponent with both of them for a quick 2-hit combo!

And, if you can back your opponent in the corner he won't slide back so far when the initial hit smacks him, giving him less time to recover and avoid the second hit.
Since this is kind of tough to show the timing in picture form, I've created an animated .gif here(4.5MB) that shows it in action.
Oh, and it also works for Street Fighter 4.
In some of the Capcom-branded fighting games, the rigmarole you have to go through to select a secret character can be quite complicated. But it isn't always.
Take X-Men vs. Street Fighter, for example.
All you have to do is go to the top row of fighters, any of them will do.

And then press Up on the joystick.

And Akuma is yours for the choosing!
What could be simpler?