Mention a bird so skillful at imitating sounds and the parrot always comes to mind right away. However, the ability to mimic sounds is definitely not exclusive to parrots.
As it turns out, renowned vocal mimics in the bird world include “mockingbirds, starlings, mynahs, marsh warbler, lyrebirds, bowerbirds, scrub-birds and African robin-chats”. Read more...
The Superb Lyrebird flaunts its incredible adeptness towards mimicry, imitating a Kookaburra, a camera shutter, a car alarm, and a chainsaw!
Watch "Lyrebird”, Youtube.
The Superb Lyrebird (Menura superba, or M. novaehollandiae) is an Australian ground-dweller “named for the shape of their tail when spread in courtship display. The name also aptly suggests a musician”. Read more...
So what inspires the Lyrebird to become such a successful impersonator?
As David Attenborough clearly describes, the male Lyrebird “incorporates other sounds he hears in the forest into a complex song in attempt to out-sing rivals in his courtship display”. As if copying the songs of other bird species lacks some kind of star performance, it extends its repertoire to include the reproduction of man-made sounds.
The Lyrebird goes this extra mile so as to win the attention of the female bird, drawing her into his territory to admire not just his elaborate song, but also his attractive plumes.
The level of mimicry mastered by the Lyrebird is a fascinating illustration of the role of learnt behaviours in social communication.
But what mechanism underlies this ability to produce such complex sounds? 
“The muscles of the syrinx (sound-producing organ in birds) control the details of song production; birds with more elaborate system of vocal muscles produce more complex songs”.The Life of Birds by David Attenborough. As it turns out, the lyrebird's syrinx is the most complexly-muscled of all songbirds. Read more...
In addition to its impersonations of other species’ songs, it learns and incorporates the new sounds it is exposed to such as the camera shutter and the car alarm. So while the Lyrebird’s instinctive loud bird song is a product of nature, its integration of other interesting sounds into its song are inspired and nurtured by sounds it hears in the forest environment.
And the adaptive significance of this learnt behaviour in Lyrebirds is clear – to increase its attractiveness and, the chances of mating and reproduction. Large song repertoires increase a male's attractiveness to females.
An interesting question to ponder on: How does the female Lyrebird is able to differentiate the male's "performance" from the actual production of these sounds sounds (e.g. camera shutter, car alarm going off). To the regular hiker making his way through the forest, what may sound like a whole range of birds singing and the sounds of foresters working close by may well be just one "liar" bird's deceptive song.
One wonders, what else does this “liar” have up his sleeves?
Of course, we will always remain intrigued at the ability parrots have to imitate human voices. 
Michael Schindlinger’s article “Why do parrots have the ability to mimic?”, lends insight into this amazing ability.
“When parrots are kept as pets, they learn their calls from their adoptive human social partners. Part of their appeal as pets is their ability to sing lower notes than smaller birds and so better reproduce human voices. In the wild, though, their calls may go much higher in pitch and much faster in tempo than any human tutor's voice.”
Once again, the role of environment in learnt behaviours demonstrated.
The benefits of birds’ reliance on learning for vocal development are also described in this account. Read more...
“Some benefits of learning may include development of context-specific calls. Imitative vocal learning is also a reliable social display of neural functions—requiring good hearing, memory…” - qualities even females humans seek out in her mate!
References
“Bird Songs”, by Gareth Huw Davies. The Life of Birds by David Attenborough. Retrieved 14 Mar 2008.
“Lyrebird”, Encyclopedia Britannica - the Online Encyclopedia. Retrieved 17 Mar 2008.
“Lyrebird, From Wikipedia, the free encyclopedia”. Retrieved 17 Mar 2008.
“Why do parrots have the ability to mimic?”, by Michael Schindlinger. Scientific American, 5 Dec 2007.