Megalodon

Ah the Discovery Channel, where would we be without you? This channel, with its, we admit, overestimated love for documenting the killing power of sharks and other predatory fish has managed to push forward some of the greatest B-movie fodder of our times. They shone the spotlight on our old friend Jaws, singled out the flesh eating piranha swarm, untangled the truth about the Giant Squid and now it has brought forward a Sea Monster for the new millenium: the Megalodon.

It's basically just a big frakking shark. Carcharodon megalodon (or possibly Carcharocles megalodon - see the other wiki for details) lived from 25 to 1.5 million years ago, grew up to 20 metres at its most favourable estimates, was an opportunistic predator and is believed by some to have died out because it couldn't compete with orca whale who could handle the cold waters better. However in low grade schlock movies it will be uncovered after being frozen in ice, go on a superhunting rampage, tracking down our intrepid heroes all over the world before eating Boeing 747's in a single bite while flying.

The appeal is simple: being prehistoric it is a bit like a dinosaur and Jurassic Park was cool, it is a shark and Jaws is not only cool but the Trope Codifier for creature attack movies and it was the biggest of its kind just like Steven Spielberg's Oscar cabinet. Evidently, this generation is the one who became film directors because of Steven Spielberg.

See also Everything's Even Worse with Sharks. Many cases overlap with Kraken and Leviathan. Compare to Giant Squid. Probably deserves honorary mention among Stock Dinosaurs, though nobody would mistake it for one.

Examples of Megalodon include:

Comic Books

Megalodons are briefly seen living in the waters near Themyscria during Gail Simone's run on Wonder Woman. Steve Trevor reacts appropriately when one lunges at him.

Film

Mega Shark vs. Giant Octopus
Shark Attack 3: Megalodon.
Megaladon: A 2002 shark film about oil drillers near Greenland who uncover a HUGE deposit of surviving prehistoric animals(including the Megaladon) which goes on a killing spree and even crashing an airborne HELICOPTER from deep below the surface which singlehandedly ruins everyones day. At the end of it all one of the characters makes a Heroic Sacrifice and the survivors get away.....until ANOTHER one pops out
In Shark Hunter, the main character's parents are killed as a kid by a Megalodon. As an adult, he joins a team going into a trench to figure out the fate of an underwater research station. Well turns out, it was a Megaldon which ends up assaulting the sub and causing general death and destruction.

Literature

Megalodon by Robin Brown came out in 1981, before the prehistoric megashark became a household name (resulting in the novel being republished under the uninspiring name of Shark) and is still one of the better efforts. The idea of Megalodon being a shallow water predator is lampshaded, with the book detailing how they've evolved to survive in a deep-sea trench - basically they've become bottom-feeders scavenging off waste floating down from above, with eyes so adapted to the darkness the Megalodon can't go anywhere near the surface without intense pain. Unfortunately the US Navy is sending down submarines to survey the trench with intent to mine its rich load of gold and uranium, at the same time that the Megalodon family gives birth to a more hungry and aggressive offspring.
Steve Alten writes a lot of books with these; his novel Meg was about a Megalodon that makes it to the surface after surviving down in the Mariana Trench. Hilarity Ensues.
Extinct by Charles Wilson (one which was almost made into a TV miniseries)
Threat from the Sea Forgotten Realms trilogy features an ancient were-Megalodon. Among his earlier achievements, this guy was the reason why the goddess of Toril's seas Umberlee got temper earning her nickname of "The Bitch Queen" and before that, he was a favourite of the shark god. Long after he vanished, most undersea sentients retained legends of him with sinister nicknames such as "The Taker", and prophecies about his return that roughly amount to Armageddon - and he was messily proving them correct all over three books and a tome of side stories.

Live Action TV

A Megalodon features in two episodes of the Walking with Dinosaurs spin off Sea Monsters.
Megalodon gets it's own episode in the National Geographic Channel series Prehistoric Predators.

Music

Mastodon's second album, Leviathan, particularly the song "Megalodon".

Tabletop Games

In Chaosium's Stormbringer supplement Demon Magic, the adventure "Sorcerer's Isle" had a megalodon that could sink ships by biting through their hulls and a giant whale-like demon named Lvthn.
Megalodons appear in White Wolf's Scion as part of the monstrous fauna the Drowned Road, Titan of Water, controls.

Video Games

Scribblenauts has a Megalodon as one of its summonable creatures.