Zerg:
Dark Swarm: The Defiler never takes showers. Because of this, he has zerglice all over him. If he sneezes, the lice can go into the air, and deflect other people's bullets. Note that firebats' fire just cuts right through the lice so they do full damage under swarm.
Plaguuuuuuuuu!: The Defiler can also vomit and spray his junk all over the enemy, making them take damage.
Consume: All McDonalds restaurants are controlled by Terrans, so the Defiler has to settle on other Zerg to keep him energized. I really love to think that Plague is just 3 half-digested zerglings being sprayed at the enemy.
Consume Addendum:The Defiler is anorexic, so you have to pay him minerals via research before he can use Consume properly.
Plaguuuuuuuuu! Addendum: And this would make him bulimic as well.
Burrow: Zerg ground units can dig a hole into literally any surface, including a freaking space station. Well, except the Ultralisk. He's too fat.
Infest Terran Command Center: Each Queen has built-in hacks that give free Infestation upgrades to Terrans within.
Parasite: Each Queen also carries portable maphacks that can be used on enemy units or critters.
Ensnare: Each Queen has a third hack that slows other units down to Normal speed while everyone else is on Fastest.
Spawn Broodling: Queens love to hack. So they got another ability: Kill any unit instantly. Of course, this doesn't work on spaceships because spaceships are too fast, and doesn't work on robots because robots have built-in antihack.
Terran:
Heal: The Medic can heal biological units by looking at them. That's how hot she is.
Restoration: As well as removing any ailment that units have.
Optical Flare: Finally, if the Medic pulls up her shirt, she can blind enemies.
Cloaking: Ghosts and Wraiths are jerks. You can tell it by their voices, as well as if you click on them enough. So leave it to them to shoot defenseless enemies while invisible.
Lockdown: The Ghost's DIY rifle can also completely fuck up the entire circuitry of any unit. The circuitry can still be healed by the Medic, however, because the circuitry loves internet porn.
Nuclear: In 500 years, the world leaders will say, "FUCK NUCLEAR SAFETY. MAKE BOMBS. NOW."
Yamato Cannons: The only thing more badass than "BADDLEKRUZER OPERAYSHUNAL" is being able to do 260 damage in 3 seconds.
Defensive Matrix: And the only thing sadder is having 250 of that damage absorbed by a stupid shield.
Irradiate:You know how when you take a shit on an airplane, the sewage is stored until the plane lands? Well the guys up at the Science Vessel just use a hatch that spills it all on enemies. Oh, and the sewage is radioactive.
EMP Shockwave: Takes out shields and makes spellcasters tired. 'Nuff said.
Stim-Pack: I actually have a problem with this one: How do steroid injections increase the fire rate of an automatic rifle?
Protoss
Psionic Storm: When the High Templar rubs his massive balls together, they create lightning that kills anything in its path.
Hallucination: He can also throw a bong at the enemy, and make the enemy think there's more enemies, and actually fire at them.
Archon Meld: If the need arises, two Templar can sex and make an Archon in 20 seconds.
Dark Archon Meld: Dark Templar saw this and were like, "WTF why do they get to have sex in the middle of battle?" They tried to do the same, and the Dark Archon is red from the internal bleeding.
Stasis: What, your spaceship can't trap any given unit inside a giant icecube?
Recall: It's my army and I want it now! With those words, J.G.Tossworth configured all Arbiters to make the Zerg rush look slow by comparison.
Feedback: Dark Archons can get hungry, and so, they feed off of others' energy. I mean, Science Vessel, do you really need that 75 points to drop your radioactive shit on my forces?
Mind Control: It's really easy to be tricked into taking orders from a giant ball of gas.
Maelstrom: Like Lockdown, but it works on biological units and crappier.