Do you ever get those nasty little black flies on your houseplants? Clouds of the buggers fly up whenever you disturb the plant. They're really gross.
They're called sciarid flies, fungus gnats or mushroom flies. And you've only yourself to blame; they eat rotting vegetable matter, such as the rotting roots of your chronically over-watered houseplants. The flies don't do any damage to your plant. If an infected plant dies it's usually from the over-watering that attracted the gnats in the first place. The flies are the symptom, not the cause.
Fortunately, the treatment is simple and completely organic. First of all let the poor old plant dry out. It won't harm the plant to do this, in fact it will probably do it a power of good. But mainly you're trying to disrupt the gnats' life cycle. The eggs take about a week to hatch, so if you can keep the soil dry for a couple of weeks you should be able to kill the already-hatched larvae by desiccating them, any larvae that hatch from the eggs after a week will also desiccate, and the adults will have no rotting roots to feed on so they should die too. So you can see that two weeks of dryness should sort the problem out.
If you feel you need an extra line of attack you could get some sticky yellow traps from your garden centre. I don't think they have a proprietary name, just ask for "sticky yellow fly traps for houseplants". These attract the flies - apparently it's the yellow colour that attracts them, not any chemical or scent or anything. And the stickiness is just glue that sticks the flies down and traps them - there's no pesticide in the traps. So this is an organic approach that should help get rid of the nasty little blighters.
Once you've eliminated the flies you might want to put a physical barrier on top of the soil to deter them coming back. An inch-thick layer of sand or fine gravel will prevent females from laying their eggs in the soil.
Finally - STOP OVER-WATERING YOUR POOR PLANTS! Always feel the soil before you water them and if the soil is moist then don't add any more water. Don't let the plant sit in a saucer full of water all day, or worse still for days and days. If there is water in the saucer half an hour after watering, pour it away. Different plants need different watering regimes, so don't water all your plants at the same time. If your moisture-loving Boston fern is looking crinkly round the edges then give it a drink, but you shouldn't water your drought-tolerant Easter cactus at the same time every time. Plants in great big pots need less frequent watering than plants in little pots, although obviously they need more water each time.
Good luck with the sciarid flies, if you have them. And if you don't, give yourself a pat on the back for not over-watering your plants.