A large and growing variety of tools can support all kinds of UML modeling aspects: from model creation to advanced round-trip engineering of UML models and code. However, such tools aim at supporting specific life-cycle phases, but they often do not meet basic requirements arising in heterogeneous environments, UML education, early life-cycle phases, or agile processes: hassle-free tool deployment, support for fast model sketching, and flexible graphic export features. This paper presents the freely available modeling tool UMLet we designed to specifically address these basic issues. It is a flyweight Java application that can easily be deployed in various development environments; it features an intuitive and pop-up-free user interface, while still providing output to common high-quality publishing formats. Thus, the tool UMLet provides an effective way to teach UML and to create and share UML sketches, especially in agile environments and during early life-cycle phases. Its user interface supports intuitive and exploratory modeling, its architecture makes distribution and deployment cost-efficient in heterogeneous environments.