an Open Source BPMN 2.0 / DMN 1.1 Modeler
MDG for Sparx' Enterprise Architect to Create TRAK arch. descriptions
A free cross-platform editor for the DRAKON visual language.
A PlantUML editor written in Qt4
Code generation from UML to any language (Java, Objective-C, PHP...)
Java library implementing a state machine by the UML standard.
PetriNetExec a library for embedding Petri Nets into Java applications