Quake 3 's InvSqrt

float InvSqrt (float x){

float xhalf = 0.5f*x;
int i = *(int*)&x;
i = 0x5f3759df - (i >> 1);
x = *(float*)&i;
x = x*(1.5f - xhalf*x*x);
return x;

}

Math explanation here (PDF)