Original HALO Warthog or Scorpion would be rather handy in a lot of places. Withered Beach pwnd, L4D pwnd, Platinum Legion pwnd, Age of Empires pwnd, Conflict Desert Storm II pwnd, Baldur's Gate pwnd ...
Short of that, the Thunderbolt from original Quake into Asheron's Call, because lightning is what works best against the Seanchan racist royalty-obsessive slave-driving invaders who think they're entitled to rule the world and own us all and sneer very rudely at anyone who isn't one of them.
Half-Life beam or Goldeneye proximity mines into any MMORPG quest with too many spawns on too short a timer.
Angband rod of wall-building into real life. I'd build myself an island ... Nothing disables an aircraft carrier quite like 10 x 10 x 100 feet of basalt.
Enemy Territory artillery support into Age of Conquerors. Trebuchet? What trebuchet?
Age of Conquerors Missionaries into real life. Just convert everyone from the Mediterranean Sea to the Bay of Bengal, and suddenly life is good.
Fully-upgraded Cobra Mk III into any Star Wars space-combat game. Star Destroyers think they've got us outnumbered? Negative, Ghost Rider. The pattern is full.
X-Com 2 : Terror From The Deep's Devastator into ... well, a torpedo you can pre-programme to fly through the corridors and smack the main target without you having to fight your way there has to be useful somewhere, right?
The silenced sniper rifle with the thermal-imaging scope from Return To Castle Wolfenstein into Morrowind.