If your home is equipped with a central air conditioning system you may have noticed a drain line on the outside dripping water near the air conditioner. This line drains condensation from the inside air handler to the outside. If this line clogs (usually from algae) it can cause water damage inside the house.
You can prevent a clog by simply pouring a cup of household bleach into the line from the inside. To remove a clog once it has occurred, attach a wet/dry vacuum to the drain line outside (removing the paper filter first) and suck the clog out.