UMLet is on open-source Java-based UML tool designed for teaching UML and for quickly creating UML diagrams.
It supports concepts like Martin Fowler's UmlAsSketch[1]. Its design goals are described in the paper "Flyweight UML Modelling Tool for Software Development" [2], Proc of 29th EUROMICRO Conference. Another paper compares UMLet to Rational Rose: "Explorative UML Modeling - Comparing the Usability of UML Tools" [3], Proc of 9th International Conference on Enterprise Information Systems (ICEIS 2007).