Despite their considerable dissemination, existing UML modeling tools suffer from significant limitations that stand in the way of their profitable use in practice as well as in teaching. This paper presents a new UML modeling tool, called UML-MX©, that overcomes these limitations. It is based on a language architecture that not only enables the integration of class and object diagrams, but also the execution of objects in the diagram editor. Thus, it promotes a more inspiring learning experience. At the same time, it goes beyond the limitations of traditional approaches to model-driven software development by enabling a common representation of models and programs.