Webgraphviz - WebGraphviz is Graphviz in the Browser
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Penpot - Design freedom for teams
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.
WebSequenceDiagrams - WebSequenceDiagrams is a web-based application that allows the user to use a proprietary language to define sequence diagram.
draw.io - Online diagramming application