Pineapple Surprise

The pin is pulled from a grenade on a grenade belt, causing the wearer and anyone in the immediate vicinity to explode. It can be used to elaborately kill a tooled-up enemy or as a makeshift suicide belt. Expect it to be used on a nearly triumphant enemy leading to a cathartic Oh Crap moment for the villain before he explodes. A subtrope of Why Am I Ticking?

Examples of Pineapple Surprise include:

Anime

At the beginning of the final battle between Accelerator and Kihara in the second season of A Certain Magical Index, the former takes out three mooks with this trick. The only remaining one promptly craps himself and tries using the Damsel in Distress (Accelerator's Morality Pet) as a Human Shield. Accelerator doesn't appreciate it.
Played for laughs in the Kochikame manga. Military crazed cop Volvo Saigo who's armed to the teeth have accidentally released his pins when fastening his seatbelt and from swimming. Since this is a cartoon, he gets better.

Comic Books

Happens to a mercenary in Danger Girl and the Army of Darkness when his possessed hand pulls out the pins of the grenades he is wearing on his vest.
In the Six Guns mini-series, the Two-Gun Kid shots a flashbang off the vest of mercenary, causing it to detonate.

Film

Leon: Leon defeats the main villain using the suicide version of this.
Raising Arizona: Hi steals the pin of one of the Badass Biker's grenades.
The Dark Knight: The Joker threatens to do this to escape a room full of mob bosses.

"Let's not 'blow' things out of proportion..." (shows thumb attached to a string, connected to several grenade pins inside his suit)

Tank Girl: The title character does this to the Water and Power mook who captured her, blowing him to bits.
X2: X-Men United: Magneto pulls the pins from a squad of soldiers all at once using his powers.
Tom Cruise's The War of the Worlds has the main character pull the pins of at least one while he was being sucked into one of the tripods. Since he spit them out when his cagemates pulled him back, it seems implied that he pulled them with his teeth.
I Am Number Four: Number Four kills the leader of the Magadorians by using his telekinesis to set off all of the leader's futuristic bullets while they're still strapped to his chest.
Captain America: The First Avenger: Captain America pulls the pin on a grenade being carried on the back of HYDRA bike during a bike chase.

Literature

Doc Sidhe: Doc does this while possessed by the spirit of the Warbringer; using magic to cause the grenades Blackletter's men are carrying to detonate while they are still wearing them.

Video Games

Call of Duty Black Ops: During "S.O.G.", an NVA soldier pounces on Alex Mason. After the player struggles for a bit, Mason pulls the pin on one of Charlie's grenades and pushes him away.
- This foreshadows a moment at the end of "Payback": after Frank Woods stabs Lev Kravchenko, Krav pulls the pins on all of the grenades in his bandolier at once. Woods tackles him out the window and they explode off-camera.
- It's also a reference to Call of Duty 3, wherein the exact same thing happened between the player and a Nazi soldier.
In Vampire: The Masquerade Bloodlines "Nines" Rodriguez saves the PC from Sabbat goons by threatening them with a grenade. Given Nines is at a power level to curb stomp the 3 young Sabbat easily, you seem to be the only reason he needs to use such a card.
Invoked In Name Only in Marvel vs. Capcom 3 by Deadpool as one of his special attacks. He tosses the grenade as he jumps away from the target yelling the trope name.
A possible tactic in Velvet Assassin, although it's a Stealth Based Game so it might be more trouble than it's worth.
In the third Uncharted game this is possible to do when engaging in melee. Just make sure you get away in time.
One of the ways to complete "I Put A Spell On You" for the Legion in Fallout: New Vegas is to pull the pin on Davey Crenshaw's grenade and later use his pranking nature to convince Colonel Hsu that he planted the bomb on the NCR monorail.

Web Original

The title card for the Nostalgia Critic's review of Commando shows the critic holding four of Arnold's grenade pins, with him freaking out in the background.

Real Life

U.S. Senator Max Cleland ended up being a real life case of this when a grenade whose pin had fallen out shredded both his legs and one of his arms during the Vietnam War. It turned out that the grenade came from a squad mate who had straightened all of the pins on his grenades (a dangerous but common enough practice during the war), but had failed to secure them with duct tape.