Neat Inverse Square Root Function

Found this nice function in slashdot: http://www.beyond3d.com/articles/fastinvsqrt/

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;
}