Here's my entry at 79 bytes. This is admittedly my first golf, so can probably stand some improvement.... It uses a probablistic method for determining pi. As tested below its usually accurate to 2 or 3 decimal places, but is sometimes worse. Accuracy can be increased by increasing the number of iterations.

for(1..999999){$x=rand();$y=rand();$k++if sqrt($x*$x+$y*$y)<1}print 4*
+$k/999999
[download]

I managed to get it down to 61 bytes.... I'm still humbled by some of the other submissions....

for(1..1e6){$k++if sqrt(rand()**2+rand()**2)<1}print $k*4/1e6
[download]