In the Uncle Grandpa episode "Uncle Grandpa Ate My Homework", after saving his class from a mummy Dennis gets an A+++++-+.
The plot of Avatar: The Last Airbender revolves around this. Fire Lord Ozai is the most powerful firebender in the world. But on the day of Sozin's Comet, every firebender will be much stronger than usual, which allows him to burn the world! He forgot that the Avatar was also a firebender, among other things.
In the Blaze and the Monster Machines episode "Truck Rangers", the volume knob on Blaze's amplifier used to stop Crusher's Robo-Bear from stealing the Super Bravery badge goes up to 11. When he turns it up to such, the volume becomes so loud it makes the Robo-Bear listen to him completely and returns the badge.
In an episode of ReBoot, Megabyte gatecrashes a concert and produces a powerful electric guitar. Unsatisfied with the sound after a couple tests, he reaches for the dial and cranks it up to 11.
At this point Bob jumps up on stage, whips out his own guitar, and the two of them have an epic rock-off. Megabyte then says "I've always wanted to do that", gives Enzo the guitar as a birthday present, and drives off in his stretch limo.
In The Powerpuff Girls' episode "Bubblevicious", Bubbles got sick of being the "kid" in the group, so she turned the training simulator up to eleven to prove she was "hardcore". The result is a Moment of Awesome for her and, while she takes it a bit to far, her sisters acknowledge it once she defeated Mojo Jojo by herself, after withstanding a torture laser at its highest level.
Buttercup: Wow, she took the laser all the way up to 11!
In The Simpsons, Pete Townshend (Guitarist for The Who) has an amp with a setting in place of 11 reading simply "Whuh-Oh", that's strong enough to partly demolish a wall made of solid garbage.
Mr. Burns hires a hypnotist to motivate his softball team.
Hypnotist: You are all very good players.
Team: We are all very good players.
Hypnotist: You will beat Shelbyville.
Team: We will beat Shelbyville.
Hypnotist: You will give 110 percent.
Team: That's impossible. No one can give more than 100 percent. By definition, that is the most anyone can give.
Much of the show is the titular brothers taking everything Up to Eleven, from building a rollercoaster to owning a goldfish.
Phineas: Guaranteed to kick any party up to an eleven!
In "S'Winter", Doofenshmirtz builds a laser that has a melting capacity of seven...on a scale of one to five.
In Home Movies, Duane's band SCAB scrawled the number 11 on the far end of their amps' volume dials.
In the Teen Titans episode "Only Human", Cyborg is able to exceed the limitations built into his cybernetics in order to defeat Atlas. (The fact that Cyborg's human mind and willpower - not his robotic body - is the stronger part of him is later a big plot element in the third season.)
Cyborg complains that, as a machine, he cannot go beyond 100% of his ability. He eventually goes up to 130% before the power readout breaks, and he decides not to replace it.
A literal example occurs in "The Lost Episode", where music-using villain Punk Rocket cranks his guitar up to eleven for more power.
In the episode "Out of This World" of Ruby Gloom, Frank and Len crank the volume of their music up to eleven curiously at Poe's suggestion. Cue fireworks and an explosion that sends them all into orbit around the moon.
Megas XLR is built around this trope. The guns, the robot, even the karaoke get turned to eleven regularly. Its stereo system goes up to eleventhousand.
Said stereo actually has more security preventing its use than all of Megas' weapons and is the one weapon Coup actually considers cruel to use. As if that didn't already show how powerful it is, when combined with Coup's off-key voice, it becomes a weapon of mass destruction capable of destroying a massive space station and an army of monsters and robots in seconds.
In Shaun the Sheep, whilst trying to cure one of the other sheep of hiccups, Shaun pulls out an electric guitar and amplifier and turns the amplifier up to eleven. As soon as he plays a single chord, a mushroom cloud is seen rising out of the field.
One episode of ChalkZone takes it one step further by cranking it up to 12 in a song at the end. It's part of the lyrics, even.
In the Ed, Edd n Eddy episode "Truth or Ed", when Eddy and Ed take control of the school newspaper, it's shown that Ed pulled the speed control lever on the printing press beyond its maximum setting, tearing through the outside of the press in the process, and Eddy wrote "Stupid fast!" near the new setting.
In the Rocket Power episode "Bruised Man's Curve", Otto returns to the Shore Shack after a painful day of training on the titular mountain board course. When Sam asks Otto to rate his pain on a scale from 1 to 10, Otto replies, "12."
In "The Party Animal", Emperor Awesome cranks the volume of his planet-ending party all the way up to 11.
In "The Bad Neighbors", Lord Hater's sound system goes up to 100 while Emperor Awesome's goes up to infinity.
In an episode of Inspector Gadget (2015) there is a "level eleven security alarm" in headquarters. Sound bad? Well, considering that their scale has only five levels...
In the Wabbit short "Bugs in the Garden", Wile E. pits his ACME home gardening products against Bugs' traditional gardening methods. One of them is a hormonal supplement that contains 6,984 more vitamins than a plant actually needs note If you don't know, a plant typically needs only 6, and he decides to use 100 boxes of the stuff. When they have a taste test, Wile E. eats a mutated carrot and mentions that he can taste the 1,700 vitamins it's been fortified with.
In the Mickey Mouse (2013) short "Roughin' It", Donald sets his RV's power supply to 11 to spite Mickey when he tries to shut the power off. Somehow, this brings the RV to life.
The Dick Tracy cartoon "Racer Chaser" has detective Joe Jitsu pursuing villains Stooge Villa and Mumbles, who have stolen a valuable and very fast race car. To catch up with them, Joe extends the speedometer of his car by writing increments up to "500" underneath it.
In Bounty Hamster, a villain who has body-swapped with Cassie gets some extra speed out of Marion's ship by scrawling "11" on the appropriate control (which usually only goes up to 5). This causes it to go fast enough that even Marion, who isn't exactly sane at the best of times, is terrified.