Q_rsqrt

I still remember encountering the following piece of code while browsing through the Quake 3 source code back in 2005:

float Q_rsqrt( float number )
{
  long i;
  float x2, y;
  const float threehalfs = 1.5F;

  x2 = number * 0.5F;
  y  = number;
  i  = * ( long * ) &y;  // evil floating point bit level hacking
  i  = 0x5f3759df - ( i >> 1 ); // what the fuck?
  y  = * ( float * ) &i;
  y  = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration
  // y  = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed

  #ifndef Q3_VM
  #ifdef __linux__
    assert( !isnan(y) ); // bk010122 - FPE?
  #endif
  #endif
  return y;
}

That comment on line 10 pretty much sums it up: WTF!?!?

I again ran across this piece of code while browsing an amuzing list of the best comments in source code you've encountered, over on Stackoverflow. It turns out the original author of this illustrious piece of code is still unknown (it's not John Carmack as you might have assumed). A nice write-up of the likely history of this hack can be found on Beyond3D. Interesting reading for sure!
UPDATE: The original author is actually known and it is Greg Walsh. Wikipedia also has a lot of background.

Talking about funny comments in code, my personal favorite from the Stackoverflow list must be the following:

stop(); // Hammertime!

Not so much the comment, but the german version really cracked me up!