Python (single expression)
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Now, while this is quite easy to figure out for Pythoneers, others might find it at least interesting. If you wonder why there's so many lambdas, I wanted to have maximum underscoreness, but minimal repetition. Who doesn't love underscores? [Ideone](http://ideone.com/DY6Uso)

    (lambda ______________:(lambda ______,_______,________:(lambda _____:(lambda _,___,__,____,_____________:(lambda _________,__________:(lambda ___________,____________:___________(____________,____[_________**(_________+__________)-__________::_________+_________+_________]+_[__________::_________]+__[_________+__________])(____________,______________))((lambda _,__:________(______(_))[__]),(________(__import__(_[_________]+__[_________*_________+__________]+_[_________]))[_[_________:_________+_________]+___[__________/_________]+__[__________]+_____(u'')[__________/_________]+_[_________+__________]])))((_____________(_)+_____________(_))/_____________(_),_____________(_)/_____________(_)))(_____([]),_____({}),_____(None),_____({}.__iter__),lambda _:len(_)))(lambda _:_______(______(_))))((lambda _:_.__class__),(lambda _:_.__name__),(lambda _:_.__dict__)))('Just another Python Hacker\n')

An indented version if you prefer:

    (lambda ______________:
    	(lambda ______,_______,________:
    		(lambda _____:
    			(lambda _,___,__,____,_____________:
    				(lambda _________,__________:
    					(lambda ___________,____________:
    						___________(____________,____[_________**(_________+__________)-__________::_________+_________+_________]+_[__________::_________]+__[_________+__________])(____________,______________)) (
    						(lambda _,__:________(______(_))[__]),
    						(________(__import__(_[_________]+__[_________*_________+__________]+_[_________]))[_[_________:_________+_________]+___[__________/_________]+__[__________]+_____(u'')[__________/_________]+_[_________+__________]]))) (
    					(_____________(_)+_____________(_))/_____________(_),
    					_____________(_)/_____________(_))) (
    				_____([]),
    				_____({}),
    				_____(None),
    				_____({}.__iter__),
    				lambda _:len(_))) (
    			lambda _:_______(______(_)))) (
    		(lambda _:_.__class__),
    		(lambda _:_.__name__),
    		(lambda _:_.__dict__)))('Just another Python Hacker\n')

This defines a function and calls it with 'Just another Python Hacker'. Now, it didn't have to be a function, but I thought it would be more elegant.

    f = (lambda ______________:(lambda ______,_______,________:(lambda _____:(lambda _,___,__,____,_____________:(lambda _________,__________:(lambda ___________,____________:___________(____________,____[_________**(_________+__________)-__________::_________+_________+_________]+_[__________::_________]+__[_________+__________])(____________,______________))((lambda _,__:________(______(_))[__]),(________(__import__(_[_________]+__[_________*_________+__________]+_[_________]))[_[_________:_________+_________]+___[__________/_________]+__[__________]+_____(u'')[__________/_________]+_[_________+__________]])))((_____________(_)+_____________(_))/_____________(_),_____________(_)/_____________(_)))(_____([]),_____({}),_____(None),_____({}.__iter__),lambda _:len(_)))(lambda _:_______(______(_))))((lambda _:_.__class__),(lambda _:_.__name__),(lambda _:_.__dict__)))
    f('Hello.\n')
    f('Just another Python Hacker here.\n')