https://zenuml.com/
A Free Sequence Diagram Online Tool | ZenUML
Description: JavaScript based diagramming tool that renders Markdown-inspired text definitions to create and modify sequence diagrams dynamically.
sequence diagram onlinefreetool
https://sequencediagram.org/
SequenceDiagram.org - UML Sequence Diagram Online Tool
Free sequence diagram online tool. Create sequence diagrams using textual notation or draw quickly via Drag and Drop with an easy to use interface.
sequence diagram onlineumltool