[lmikle]

Можно еще хитрее:

Код:

function BitCount(B : Byte) : Integer;
const
  cMask : Array [1..8] Of Byte = (1,2,4,8,16,32,64,128);
begin
  Result := (B And cMask[1]) +
               (B And cMask[2]) +
               (B And cMask[3]) +
               (B And cMask[4]) +
               (B And cMask[5]) +
               (B And cMask[6]) +
               (B And cMask[7]) +
               (B And cMask[8]);
end;