Modelio - Modelio is an open source modeling tool supporting natively UML, BPMN and model exchange with XMI.
Invision - Prototyping and collaboration for design teams
Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
Zeplin - Collaboration app for UI designers & frontend developers
UML Designer - UML Designer provides a set of common diagrams to work with UML 2.5 models.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.