I've just made a simple function for RGB to HEX conversion, and since I have nothing else to do, I'll go ahead and post it. I'm doing it mainly because I've seen some of the other ones, and they can take up to 50 lines. This one takes up around 12.

function toHex(_red,_green,_blue) {
 var _hex=new Array('0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F');
 var _color=new Array(_red,_green,_blue);
 var _code=new Array("","","");
 for (i=0;i<3;i++) {
  //High bit, low bit
  _fbit=(_color & 0xF0) >> 4; //Four-bit value, 0-15
  _code+=_hex[_fbit];
  _fbit=_color & 0x0F;
  _code+=_hex[_fbit];
 }
 return _code[0]+_code[1]+_code[2];
}

There are probably some other ways to make it small, but I can't think of any right now...