Those are "moth flies", also known as drain flies.  The immature stages are worm-like, long and thin compared to their width, usually darkly colored, and semi-aquatic.  In buildings the immature stages live in drains, especially floor drains or any drain that does not get a lot of use.  In drains, they live at or just above the water line in the drain trap.  A occasional potful of scalding hot water poured very quickly in to the drain will kill a good portion of them.  Running a drain cleaning brush down through the trap and then running a great deal of water through the drain will also work.

There is just one other place that I have seen drain flies thrive-- in "worm composting" bins, if the system is kept just a bit too moist.