I always love implementing this (++ if you can tell where the algorithm comes from!) 
for($|++,$_++,$a++;;$,+=$a/$_,$_++,$a*=-1,$_++){printf"%.16f\n",$,*4}

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This version will run (incessantly) as a one-liner as well. -zeno

Update: As MrNobo1024 has answered correctly, this algorithm was discovered by Gregory and Leibniz.

In reply to one-liner pi by zeno

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