PlantUML Online Server
plantuml.com
Renders UML from a text description, with the source encoded in the URL
No sign-up
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sequencediagram.org
Purpose-built editor for sequence diagrams, typed rather than dragged.
A single-format tool that does sequence diagrams better than the general ones do: readable syntax for participants, activations, loops, alternatives and notes, with the rendered diagram updating as you type and clickable elements that jump back to the matching source line. It exports SVG, PNG and PDF, and saves to your device. It draws nothing else — no flowcharts, no ER diagrams — which is the trade that makes it good.
Questions
Works the moment the page loads. No account, no email, no trial. Sequence diagrams only — it will not draw any other diagram type.
No. SequenceDiagram.org does the work inside your browser, so the file never leaves your device — you can load the page, disconnect from the network, and it still works. That makes it a reasonable choice for material containing personal data.
Sequence diagrams only — it will not draw any other diagram type.
Alternatives
Same category or same task, ordered by how closely they overlap.
plantuml.com
Renders UML from a text description, with the source encoded in the URL
diagrams.net
Full diagram editor that works without an account or a saved file
mermaid.live
Write a diagram as text and watch it render beside you
asciiflow.com
Draws boxes and arrows as plain ASCII you can paste into a comment
excalidraw.com
Hand-drawn-looking whiteboard that opens instantly with no sign-up
lucidchart.com
Polished diagramming with real-time collaboration and smart layout