The comic has milked a lot of humor from Gwynn's violent tendencies, including how often she nails Riff in the groin when he acts like an insensitive jerk. Interestingly enough, the strip on August 21st, 2007 shows that Zoe's just as affected by a crotch kick as the two male leads.
In 8-Bit Theater Black Mage, soon after becoming a Blue Mage (which allows him to learn attacks he survives getting hit with) is hit by a "Goblin Punch", which is actually just a kick in the nuts from a non-goblin monster (the attack was named by "the greatest cryptozoologist in all Red Wizard history, Blindy O'Sightless"). He learns this attack and returns itwith enough force to launch the enemy into low orbit.
In Monsterful, the mummy Lily Osiris has shown her pervert ghost friend Lawrence that not even ghosts are safe from this.
In a storyline of Nip and Tuck, Thelma Opossum was about to be photographed in a bikini when the photographer tells her that she have to lose the outfit in question for a nude photo shoot. She immediately starts kicking lecherous guy's butt, which gets the attention of a lady wrestler.
In Questionable Content Faye accidentally headbutts Marten's groin and then vomits all over him while she's blind drunk.
In a later comic, the Vespavenger's Vespa, which could morph into a not-so-giant-mecha, manages to also hit Marten in the crotch, with a metal pole. This leads to the statement from Marten as he's on the ground in pain, "Pain level...Shatnerian... in...intensity"
Asperchu receives one of these in his titular webcomic.
Casey and Andy. Casey picks on a short alien with a single eye on a stalk. The alien retaliates. Casey spends the rest of the day watching Independence Day. On the other hand, when Quantum Cop catches the aliens and they give him the Groin Attack, he's wearing a cup. And he writes them a citation, since he happens to know their legal system too.
Yuki of Ménage à 3 is the daughter of a manga-ka who neglectfully exposed her to his tentacle porn work at a young age. As a result, whenever she sees or feels a man's penis, her mind registers it as a set of threatening tentacles, usually causing her to go temporarily insane, scream "TENTACLLLLES!" and kick the unfortunate fellow in the groin. She has even gone on extended cross-town groin-kicking rampages when particularly upset. Therapy is slowly resolving this problem, but she's not safe yet.
In User Friendly, there is a "Forced-Feedback Enemy-Denial Smackdown Ergonomic Gaming Chair", which features a boxing glove on a stick aimed at the face for when your character is gibbed, and a metal, spike-knuckled gauntlet on a hydraulic ram positioned directly over the crotch for when your character is killed by Quad Damage.
In Autobot Burnout's Funnies (a photocomic on Transformers World 2005), a recent (as of February 3rd, 2011) arc has Battle Ravage defeat the Fallen this way, using his (Ravage's) girlfriendTigris' Buster Sword. As a result, The Fallen ends up with a censor bar over his gearbox.
In Homestuck, a newly undead Kanaya kicks Gamzee in the crotch so hard it knocks him out of his shoes and off a cliff.HONK!
Female Example: Nosh delivers one to Sanny after the latter strikes a nerve in the stalled-out webcomic For Your Eyes Only. Ben then mentions how the trope normally applies to girls as Sanny writhes in pain.
In Beyond the Canopy, young Glenn punches a much larger bully, Thistle, right in the acorns. When Glenn meets Thistle again some years later, he's wearing an armored codpiece to prevent further injury to that region.
In the Sister, Sister arc of Elf Blood, Fliss delivers one to the female Death Elf punk. It has no effect, much to Fliss' protest: She's been kicked there before and it REALLY hurt.
Greg takes one in the family jewels as he gets mistaken for a stalker. He's down for the count.
In San: Three Kingdoms Comic, this is the ultimate attack that Zhuge Liang refuses to learn from his teacher. Same goes to Sima Yi.
Also in the Ask Cao Cao segment, Cao Cao got hit on the groin while playing tennis with Cao Ren, resulting him to order Cao Ren's castration and starting a long line of Running Gag of "Let me goooo I'm still virgin..."
In Schlock Mercenary, an ore freighter's captain is praised for taking out a warship with just "rocks, ramming and balls of steel". He proceeds to clarify he had lost that last part of his "inventory" in a long-ago war.
Dr. Bowman, in the July 28, 2014 strip of Freefall, mentions that in the past he took a sharpened plastic spoon and neutered himself after seeing what testosterone-fueled aggression was doing with other Uplifted Animals, which ultimately led to their deaths at age 40. Florence asks him if they can skip the details of the procedure, and move on to her next question for him.
In The Young Protectors, Fluke has the distinction of being the only known mortal to stab a Demon Lord in the nethers. Given that the demon is a solid 60 feet tall, it's more distracting than debilitating, but still...