When you think of flies, its normally the boring black houseflies or even worse those nasty biting horse/deer flies that come to mind. But other flies are actually quite beautiful and interesting. Flies are of the order Diptera, which means two wings.
This is a robber fly, in the Asilidae family. This particular one was pretty tiny. In general robber flies will attack other kinds of flies, beetles, dragonflies, and the like. A quick google turns up all kinds of critters that various robber flies have conquered.
Next is a hover fly, the Syrphidae family. They are generally harmless, mostly feeding on pollen and nectar. They have so much great color and detail in their abdomens, which are designed to mimic wasps and bees as a defensive tactic.
Here is a bee fly, family Bombyliidae. This is another harmless nectar eater, superficially looking like a bee, a cute fuzzy little bee.
All of these were shot near Berthoud, Colorado, the last two with a 28-105mm lens (with an ultrasonic focus, so it locks on fast which is a big help for these) and some combination of +2, +4, or +10 close-up filters, the on-camera flash even though it was full daylight, and the tightest aperture available (f/25), from a distance of about 3 inches. The robber fly I somehow lucked out and got it from a couple feet away with a long lens.