Webgraphviz - WebGraphviz is Graphviz in the Browser
Penpot - Design freedom for teams
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
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.
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.