draw.io - Online diagramming application
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
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.
Sketch - Professional digital design for Mac.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Adobe Illustrator CC - Adobe Illustrator CC is a commercial vector graphics editor.