Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
Modelio - Modelio is an open source modeling tool supporting natively UML, BPMN and model exchange with XMI.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
UML Designer - UML Designer provides a set of common diagrams to work with UML 2.5 models.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.