Harassing your opponents workers can be an effective strategy in SC2 games. Successful harassing will slow down your opponents macro, giving yourself time to continue building up and going in for the kill. Here are a few harassing techniques:

Air-To-Land Harassment

Mutalisks, banshees, and void rays can all be used to knock out some of your opponents workers. The advantage of these aircraft are they are very fast. A successful harassment does not necessarily mean you kill all the workers- even killing just 6-8 of them is helpful, especially if you are able to get your units out of harms way when he sends in units that can attack your aircraft.

Transport Harassment

If your opponent does not have many defenses near his workers and not too many troops near his workers, you can use a transport to drop units, kill the workers, and then fly away to safety when his main army arrives.

Cloaked Units

Using cloaked units is generally the most effective way of killing all of your opponents workers. For Terran, this generally would be cloaked banshees (though technically you could use ghosts too) and dark templars for Protoss.

The effectiveness of cloaked harassment largely depends on your opponents detection capabilities. Cloaked harassment can often work well against Terran, since many players rely too much on the scanner sweep to detect cloaked units. When he does the scan, simply move your units away as fast as possible. Chances are he doesnt have enough energy to sustain continuous scans.

Cloaked harassment can work well against zerg if your opponent doesnt have any overseers. However, upon a cloaked rush, your opponent will likely morph one, so your harassment will be able to work as long as the overseer is in the cocoon. This still can be enough time to kill all of his workers if you have enough units.

Protoss have good detection capabilities since many players use photon cannons alot and the observer is a easy, quick unit to build. However, if your opponent does not have a robotics facility since they are heavy into void rays, you can quickly kill any photon cannon near the base (they generally will not have enough) and then continue to harass your opponent and may even be able to say gg.