draw.io - Online diagramming application
StarUML - StarUML
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
UMLet - UMLet - Free UML Tool for Fast UML Diagrams
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.