Technically a Transport

Transports. We all know them. Slow. Weaponless. Altogether, quite vulnerable. But what if you could fix that little issue? In comes this thing: a humble battlefield bus turned into a substitute front-line tank. Or gunship. And it can still drop off its passengers and cargo most of the time. But who'd want to?

Tropes used in Technically a Transport include:

Video Games

Battlefield 2142: With the Air Transports filled with various members of a squad. Most commonly being Engineers, a Medic/Soldier, and Supports.
Supreme Commander: A T1 or T2 Dropship loaded with T1 Light Assault Bots. The Cybrans can stick a T2 Mobile Stealth Field Generator with the LAB's to make it impossible to shoot down early without Omni-Sensors. In Forged Alliance,the UEF gets a T3 Dropship with a self-projected shield to jack survivability. The in-house term is "Ghetto Gunship"
Twisted Metal: Just look at half the vehicles, you'll see it.

Real Life

The AC-130 gunship. It's simply a Hercules cargo plane with guns and ammo rolled on board,pointing out the port side. The AC-47 and AC-119 having a similar history.
Maybe not "a near unstoppable force of destruction", but a technical is a pickup truck fitted with guns (Also seen in Command & Conquer Generals/Zero Hour, they upgrade increasing their strength through wreckage.)
During the American Civil War the Confederacy salvaged the USS Merrimack and outfitted it with iron plating, renaming it the CSS Virginia.

Tabletop Games

Car Wars. Big rig trucks are often armed with extensive weaponry in case they're attacked by bandits or have to engage in autodueling.
Star Fleet Battles. Many interstellar civilizations used armed transports and Q-ships to defend convoys from Orion pirates and other raiders.

Film

Star Wars. The Millennium Falcon. Originally a stock light freighter, but has since been upgraded with souped up drives, heavy-duty quad laser cannons, concussion missiles, and a sensor suite that's like seeing the future.
The Batmobiles seen in The Dark Knight Saga.
In the Dawn of the Dead 2004 remake, the survivors turn a tour bus into an armoured zombie killing death machine (complete with slots that let them use chainsaws to get rid of undead hitchers) in order to make a break towards the pier.
The A-Team with their modified vehicles.
The movie The Pentagon Wars is about a Real Life development project that went this way: the Bradley Fighting Vehicle started out as a pure troop transport vehicle, but as development continued it acquired more and more features of a tank.