Love UMLet (http://www.umlet.com/ http://www.umlet.com/), particularly for the ability to copy and paste diagrams instantly. In contrast to some others here, I

Love UMLet (http://www.umlet.com/ http://www.umlet.com/), particularly for the ability to copy and paste diagrams instantly. In contrast to some others here, I find quick, simple diagrams immensely useful in resolving long, unproductive discussions. They tend to focus discussions on concepts, ontologies and cardinalities instead of trying to verbally represent mental models.