Fungus gnats. No, they are NOT good! If the plants are indoors, a combination of yellow sticky traps placed horizontally on the edges of the pots will help trap the flies. Use a form of Bt (specfically fomulated for these) with your waterings to kill off the maggots. They also infest the soil with fungus maggots that destroy root systems and weaken plants. If they are outdoors, a Bt insecticide would help to kill off the fungus maggots, or you can use a beneficial nematode specifly meant for controlling these. This same subject has been covered many times in the past. Even though the search engine here shows many posts and replies about this same tiny bug. Worst thing is they can infest every single indoor plant you have, even if they are found in just one single pot.

...or you can stop watering as much and allow the top 1/4" of the soil dry out. Scratch at the surface to bring eggs/maggots to the surface and they'll dry up too :D
If you want to make your own yellow sticky trap to catch adults, hang a yellow piece of construction paper with vaseline smeared on it over the plant or attach it to a plastic fork and stick it in the pot near the rim.

Because the plants are probably already badly stressed, it may not be a good idea to allow them to dry out. The maggots will move closer to the roots and do more feeding. The link below has the Bt product which is very efective and will not cause any problems with herbs that are eaten by humans.
The link below has the sticky traps which should be placed horizontally on the edges of the pots. These are plastic, and can also be used outdoors as the sticky adhesive will not wash off with the weather.

http://www.gardeners.com/Whitefly-Traps/default/05-244.prd

Here is a link that might be useful: Bt insecticide

Prevention is better than cure so don't overwater. The link gives a succinct outline of the problem.

Here is a link that might be useful: fungus gnats