The best open source alternative to ArgoUML is Dia, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to ArgoUML and 19 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to ArgoUML are PlantUML (Free, Open Source), Umbrello (Free, Open Source), Papyrus UML (Free, Open Source) and UMLet (Free, Open Source).
The free to use, open source toolkit YAKINDU Statechart Tools (SCT) provides an integrated modeling environment for the specification and development of reactive, event-driven systems based on the concept of statecharts.
TopCoder UML Tool is an easy to use, consistent modeling tool for use in Design and Development competitions. The new tool was built to model sequence, class, use case, and activity diagrams. On top of that, all those elements can have documentation easily attached to them.
ClassBuilder is freeware CASE tool targeted at the C++ developer, running on Win2K-XP. It lets you create, manipulate and navigate classes, class relations, class members and class methods at a high level of abstraction through a graphical user interface.
Gaphor is a Python-based, easy to use modeling environment. This means that you are able to create nice UML diagrams for documentation and to assist you with design decisions. Gaphor will help you create your applications.