Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function RECharIndexByPos(RichEdit: TRichEdit; X, Y: Integer): Integer;
 var
   P: TPoint;
 begin
   P := Point(X, Y);
   Result := SendMessage(RichEdit.Handle, EM_CHARFROMPOS, 0, Longint(@P));
 end;


 procedure TForm1.RichEdit1DragOver(Sender, Source: TObject; X, Y: Integer;
   State: TDragState; var Accept: Boolean);
 var
   P: TPoint;
 begin
   // Check, if Source is ListBox1 
  Accept := Source = ListBox1;
   if GetCursorPos(P) then
     with RichEdit1 do
     begin
       // Get the Index from Mouse Position 
      P := ScreenToClient(P);
       SelStart := RECharIndexByPos(RichEdit1, P.X, P.Y);
       SetFocus;
     end;
 end;

 procedure TForm1.RichEdit1DragDrop(Sender, Source: TObject; X, Y: Integer);
 begin
   // finally insert text at mouse position 
  RichEdit1.SelText := ListBox1.Items[ListBox1.ItemIndex];
 end;