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.
Sankey Diagram (csaladen) - Sankey Diagram Generator
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
SankeyDiagram.net - SankeyDiagram.net is an easy-to-use tool for generating and designing Sankey Diagrams which visualize flows and especially budgets. The entered data stays in your browser and gets processed locally so you don't have to worry about privacy.
draw.io - Online diagramming application
SankeyMATIC - There are no tools to install. There is no code to write. You do not have to sign up for anything. Just enter your data, customize the diagram to your liking, and download a finished product.