Dia
Open Source
Dia is free and open source drawing software.
StarUML
Commercial
StarUML is an opensource UML tool.
UMLet
Open Source
UMLet is an open-source Java-based UML tool designed for teaching the Unified Modeling Language and for quickly creating UML diagrams.
PlantUML
Open Source
PlantUML is an open-source tool allowing users to create UML diagrams from a plain text language.
Umbrello
Open Source
Umbrello UML Modeller is a free software UML diagram tool available natively for Unix-like platforms and Windows.
Visual Paradigm
Commercial
Visual Paradigm (VP-UML) is a UML CASE Tool.
Compare ArgoUML with alternatives
Compare Visual Paradigm and ArgoUML and decide which is most suitable for you.