The cost of a special ability is the amount of energy required to cast it while the research cost is the amount of resources (minerals and gas) need to make it available.
Stim Packs
This causes Firebats and Marines to lose 10 health points but it makes the Firebat and Marine faster, fire with a higher rate of fire and allow the unit to stand a chance against most enemies. If backed up by Medics it will be useful on using the ability multiple times.
Heal
The Medic will heal any biological units including some Protoss units and Zerg units. A very useful way on making a group of Marines able to take out a strong unit like Battlecruisers.
Restoration
The Medic heals any infected units from Parasites, Blindness, Plague, Ensnare, Acid Spores, and Lockdown.
Optic Flare
This Medic ability blinds the enemy and thus reduces its sight range to zero. It also disables the unit from being a detector, such as Overlords, Science Vessels, and Observers. The optical flare makes it so that for a blinded unit to attack the enemy there needs to be another unit who spots for them. This can really be a nuisance, especially for seige tanks and lurkers, who are locked into position unless they either unsiege or unburrow, depending on the unit.
Personal Cloaking
This causes the Ghost to use energy on hiding from its enemies. Note: only detectors can spot a cloaked Ghost.
Lockdown
This causes the Ghost to do a lockdown on an enemy vehicle. Useful on fighting the Terrans and Protoss vehicles especially Carriers and Battlecruisers.
Nuclear Strike
Ghosts have the ability to launch nuclear missiles from a loaded nuclear silo. The nuke takes approximately 10 seconds to reach the location of the nuke. The ghost itself must remain completely still until the nuke reaches the location. Once the nuke is launched, there is no way of retrieving it, even if the strike is cancelled or if the ghost dies. The nuke itself does either 500 or 2/3 of the unit's HP in damage, whichever is greater. Therefore, all units but the Terran Battlecruiser (500 HP + 3 Armor) will be killed from a nuke, and most buildings will be destroyed from two nukes.
Spider Mines
This ability causes a mine to be laid in the ground but if enemies(even cloaked enemies) try to pass it the results will be the mine causes 125 damage to the passing enemy.
Siege Mode and Tank mode
The Siege tank in Siege mode becomes a long range bombardment vehicle that shells targets from afar and in Tank mode the Siege Tank becomes mobile.
Cloaking
This causes the Wraith to hide from its enemies and make the Wraith useful on fighting Battlecrusiers, Archons and most enemies without being detected. The Wraith can be detected by detectors unfortunately so watch your Cloaked units when fighting the Zerg.
EMP Shockwave
Drains all energy to zero and drains all Protoss shields to zero.
Defensive Matrix
This ability makes a unit survive more attacks.
Irradiate
This ability will kill a High Templar, a Dark Templar, a Hydralisk, a Zealot and most biological units in one use per ability. The ability does not work on Mechanical units and it causes friendly fire.
Yamato Cannon
This attack takes considerably long to charge but once it fires it does 260 damage to most enemies. This ability takes 150 energy points but its worth it. Should a target be destroyed before the Yamato Cannon hits the target it would be a waste of energy.