Magic User Interface (MUI) is an object-oriented GUI toolkit developed by Stefan Stuntz for AmigaOS that serves as a modern, customizable replacement for the standard Intuition interface. It features font-sensitive automatic layout management, skinnable widgets, and a class-based architecture using BOOPSI-like object orientation that allows developers to create complex applications with consistent user interfaces. MUI became the de facto standard GUI framework for Amiga applications in the late 1990s, offering end-users extensive customization options through its preferences system while maintaining efficient resource usage on limited hardware.