Objective Toolkit offers the following MDI alternatives and enhancements:

Although you can easily convert your MDI application to any of our MDI alternatives, you need to consider your application users when you select an interface. Our Multiple Top-Level Interface (MTI) and Floating Document Interface (FDI) are radically different from MDI, whereas our Workbook Document Interface (WDI) augments the capabilities of MDI without changing the interface drastically.