Starcraft 2 : The Reaper

The Reaper. On YouTube videos it was featured as an early game Harasser to give your opponents an itch to scratch, hopefully buying few precious moments to carry out your dastardly military stratagems.

After awhile I realised it could be a good counter attack unit, just keep it on some corner near the enemy main, and use it to attack the mineral line when he sends out all his troops to attack your own main. Hopefully by the time you repulse the attack, his harvesters would be wiped as well.