This is a drone fly (Eristalis tenax), named for its mimicry of a male honeybee.
It is the world’s most widespread species of hover-fly, found everywhere, on all continents, except the Antarctic. It is a nectar-feeder and is an important pollinator, especially now, as honey bee populations plummet.
Drone fly larvae are nightmare-ish creatures known as rat-tailed maggots. The adult fly lays its eggs in stagnant water that contains decaying organic matter, and the larvae, when they hatch, feed on the bacteria that grow there. The rat-tail, for which the larva is named, is a breathing tube; stagnant water contains little or no dissolved oxygen and the larva breathes through the snorkel that extends from its rear end to the surface of the water.
When fully grown, the larva starts looking for a drier habitat in which to pupate. The pupae, which retain the tail, look like a tiny mice; in fact fishermen, who use them as bait, call them mousies.
Check your late-flowering and winter-flowering garden plants for drone flies; the adult fly hibernates through the winter but will emerge to feed on a warm day.
Left: rat tailed maggot; right: adult drone fly