Electricity

Electricity was a form of energy. In its most useful form, electricity was what many species across the universe used for power.

Weapon
Electricity could also be harnessed as a weapon.

Biological sources
The Silents were capable of absorbing electrical energy, and harnessing it to kill their victims. Joy was killed in this way while in the White House. (TV: The Impossible Astronaut)

The newly resurrected Master was able to shoot bolts of electricity from his hands. (TV: The End of Time)

Artificial sources
The Chameleons had got ray guns able to electrocute living beings. (TV: ''The Faceless Ones)

The Slitheen harnessed electricity as a weapon during their first Earth invasion. They attached electrical transmitters to name tags and used them to zap half of UNIT's alien experts. (TV: Aliens of London)

Dæmons (TV: The Dæmons) and the Rutan Host (TV: Horror of Fang Rock) used electricity as a weapon.

Racnoss webstars could discharge electrical blasts from any of their pointed spires. (TV: The Runaway Bride)

Personally-generated electricity
The Cybermen could electrocute victims through their arm gauntlets, harming them (TV: The Moonbase et al.) or killing them instantly. (TV: Rise of the Cybermen, The Next Doctor)

Bannakaffalatta, a cyborg, had the ability to emit a high-range electromagnetic pulse. This destroyed the Heavenly Host easily, but without immediately available medical facilities, it cost him his life. (TV: Voyage of the Damned)

Davros could fire electrical charges from his hand, and then later from his cybernetic arm gauntlet. These ranged from minor to fatal voltage. He could also cut out the electrical energy of all matter using his reality bomb. (TV: Revelation of the Daleks, Journey's End)

Other uses
Some species like the Wire could physically transform themselves into electricity and travel across galaxies in that form. (TV: The Idiot's Lantern)

The Swarm infected its hosts through electrical pathways. (TV: The Invisible Enemy)

Daleks used electric barriers to protect themselves from Terrorkons while storing missiles in the Lake of Mutations. (COMIC: The Terrorkon Harvest)