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

private
 { Private declarations }
 AnchorX, AnchorY,
 CurX, CurY: Integer;
 Bounding: Boolean;
 end;

 implementation

 procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
 Shift: TShiftState; X, Y: Integer);
 begin
 AnchorX := X;
 CurX := X;
 AnchorY := Y;
 CurY := Y;
 Bounding := True;
 end;

 procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
 Y: Integer);
 begin
 if Bounding then
 begin
 Canvas.Pen.Mode := pmNot;
 Canvas.Pen.Width := 2;
 Canvas.Brush.Style := bsClear;
 Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
 CurX := X;
 CurY := Y;
 Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
 end;
 end;

 procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
 Shift: TShiftState; X, Y: Integer);
 begin
 if Bounding then
 begin
 Bounding := False;
 Canvas.Pen.Mode := pmNot;
 Canvas.Brush.Style := bsClear;
 Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
 end;
 end;