UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
draw.io - Online diagramming application
Sketch - Professional digital design for Mac.
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.
Invision - Prototyping and collaboration for design teams