Screen Tap

A character taps on your TV screen from the inside to get your attention. Yes, that's right—someone is Breaking the Fourth Wall by touching it!

As Technology Marches On, the "screen glass tap" sound effect is still in use despite being an Artifact due to the rise of flat-screen LCD's not made out of glass, though some uses now feature a "dull thump" sound effect instead to compensate for this change.

Compare with Camera Abuse.

Examples of Screen Tap include:

Commercials

Rix Fm ran a commercial in which the host tapped the screen of an iPhone.
Mr. Opportunity from Honda commercials.

I'm Mr. Opportunity, knocks on glass and I'm knockin'!

In a TV spot for Sly 2: Band of Thieves, Sly taps the screen with his cane and asks "So, uh... you in?"

Live Action TV

Channel 4 played with this trope in its promos in the early 1990s (14'14" in).

Software

The Microsoft Office Paper Clip.

Video Games

Hugo has a habit of doing this when you die in the games.
The Scout of Team Fortress 2 does this in "Meet The Scout". As a neat detail, his screen-poke leaves a greasy smudge that stays on-screen for the rest of the short.
Bubsy the Bobcat, at least in the first of his games, knocked on the screen as one of his Idle Animations. In a nice little touch, the screen even shook when he did it.
In Limbo of the Lost, the hero will tap the screen to get your attention if you spend too much time doing nothing. Justified in that, unlike most video games, you are actually a character: his guardian, who is guiding him.
In Pac-Man 2: The New Adventures, Pac-Man will tap the screen if you leave the game paused for too long.
Banjo does this in the opening theme of Banjo-Kazooie.
Sometimes, in my game Marvel vs. Capcom 3 I'll shake the screen and berate the player (that's you) for your laziness after winning a fight.
If the player idles for too long in Ken Griffey Junior's Winning Run, the umpire will turn around, tap the screen with a baseball, and say "Play the game, kid."
Simon the Sorcerer, Simon does this to show the player he is 'getting bored' when the player does nothing for too long.

Web Original

Strong Bad, in a bid to escape the 'tiny box,' actually 'breaks' your screen at the end of many of the "Podstar" versions of the episodes.
In the patient select screen of Die Anstalt, Lilo will sometimes tap on the screen with one of his blocks.
The Wiimote does this in Super 78 Studios' Wii - Hold on Tight! 2, presumably to test its durability after the previous video.

Western Animation

Tino from The Weekenders sometimes does this at the beginning of an episode, and even does it during the theme song.
"Find a happy place! Find a happy place!"
Happens on Arthur.