I just discovered this countdown of the top twenty coolest flies, compiled by Jonathan Wojcik (http://bogleech.com/bio-flies.html). It truly gives one an excellent idea of the Diptera's incredible diversity; it even brought to my attention flies of which I was previously unaware, such as Thaumatoxena wasmanni (Phoridae), the females of which are wingless, legless, mite-like indestructible lozenges that inhabit termite fungus gardens; and Wandolleckia achatinae (also Phoridae), the females of which reside as commensals on the bodies of snails, swimming in the snail-snot itself. (I've always wanted to use that phrase.)
The article also has the only photographs of the New Zealand bat fly (Mystacinobiidae)—the only social fly known to man—that I've ever seen; and there's a fantastic image of an ant-decapitating fly (Pseudacteon) bursting from its host's head. My only complaint is that the terrible hairy fly (Mormotomyiidae; subject of a forthcoming post on my personal blog) is absent from the list.