Автор: http://www.swissdelphicenter.ch

{....}

uses Commctrl;

{....}

const
  TTS_BALLOON = $40;
  TTM_SETTITLE = (WM_USER + 32);

var
  hTooltip: Cardinal;
  ti: TToolInfo;
  buffer: array[0..255] of char;

  {....}

procedure CreateToolTips(hWnd: Cardinal);
begin
  hToolTip := CreateWindowEx(0, 'Tooltips_Class32', nil, TTS_ALWAYSTIP or
    TTS_BALLOON,
    Integer(CW_USEDEFAULT), Integer(CW_USEDEFAULT), Integer(CW_USEDEFAULT),
    Integer(CW_USEDEFAULT), hWnd, 0, hInstance, nil);
  if hToolTip <> 0 then
  begin
    SetWindowPos(hToolTip, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE or
      SWP_NOSIZE or SWP_NOACTIVATE);
    ti.cbSize := SizeOf(TToolInfo);
    ti.uFlags := TTF_SUBCLASS;
    ti.hInst := hInstance;
  end;
end;

procedure AddToolTip(hwnd: DWORD; lpti: PToolInfo; IconType: Integer;
  Text, Title: PChar);
var
  Item: THandle;
  Rect: TRect;
begin
  Item := hWnd;
  if (Item <> 0) and (GetClientRect(Item, Rect)) then
  begin
    lpti.hwnd := Item;
    lpti.Rect := Rect;
    lpti.lpszText := Text;
    SendMessage(hToolTip, TTM_ADDTOOL, 0, Integer(lpti));
    FillChar(buffer, SizeOf(buffer), #0);
    lstrcpy(buffer, Title);
    if (IconType > 3) or (IconType < 0) then
      IconType := 0;
    SendMessage(hToolTip, TTM_SETTITLE, IconType, Integer(@buffer));
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  CreateToolTips(Form1.Handle);
  AddToolTip(Memo1.Handle, @ti, 1, 'Tooltip text', 'Title');
end;

{
IconType can be:

 0 - No icon
 1 - Information
 2 - Warning
 3 - Error
}