Protoss
Units and Structures:
Observer
Photon cannon
Single Player only: Purifier (Safe Haven on Level Brutal)
Terran
Units and Structures:
Raven
Missile turret
Single Player only: Science vessel
Abilities:
Scanner Sweep (orbital command ability)
EMP Shot (ghost ability) will reveal cloaked units for a short time
Zerg
Units and Structures:
Overseer
Spore crawler
Single Player only: Spore cannon
Single Player only: Kerrigan
Abilities:
Fungal Growth (infestor ability)