A Cannon is an item in Super Mario 64. To use the cannon, Mario had to talk to a Bob-omb Buddy, who would activate it. The Cannon helped him get blasted off to other places.
Although Super Mario 64 marked the first appearance of a cannon going by the name "Cannon", the Pot Cannons of Super Mario RPG: Legend of the Seven Stars previously helped Mario. Cannonball-shooting cannons of an unknown name also appeared as foes on Super Mario Bros. 3's Airships.
Cannons appeared later in Super Mario Sunshine, where Monty Moles used them. Here, they shot Bullet Bills.
A giant Cannon appears in Paper Mario: The Thousand-Year Door's Fahr Outpost level. This Cannon is big enough that it can fit more than 100 people (or at least Bob-ombs) inside of it and it can shoot anyone anywhere, even the moon.
Cannons were used to blast Yoshi to new heights in Yoshi Topsy-Turvy.
The Cannon has appeared in Mario Party 7 as an orb in Pagoda Peak and Mario Pinball Land.
A smaller Cannon that shoots Cannonballs appeared in Mario Hoops 3-on-3.
Cannons appear in boss fights against Donkey Kong in Mario vs. Donkey Kong 2: March of the Minis.
In Wario: Master of Disguise a golden cannon known as Blamses was a popular attraction.
Cannons (as well as a calm variety) are also enemies in Super Princess Peach. However, in this game, "Cannon" is merely another name for the Super Mario World 2: Yoshi's Island enemy Kaboomba.
They are not to be confused with the Bill Blaster cannons that shoot Bullet Bills.