SequenceDiagram.org is an online tool / software for creating uml sequence diagrams.

All processing of parsing and painting the diagram is done client side in the user's web browser. This not only makes it snappy to work with, but it also means that your data isn't transferred over the Internet by the tool like in many other sequence diagram tools. Since no backend server is used it is also possible to use the tool offline.

There are many sequence diagram tools out there but this is the only one which allows you to:

Script using a simple text notation when it is most convenient to do so, and
Draw the sequence diagram using your mouse at the same time when that is most convenient
In other words bringing the best from both worlds. This makes it very easy to work with large diagrams where other similar tools struggle. It also doesn't have any ads or unnecessary distracting elements.

The tool is developed, maintained, and used by people who create a lot of sequence diagrams in their daily work and from efficiency point of view weren't 100% satisfied with the existing tools.

If you have any question/comments/suggestions/bugs please contact info@sequencediagram.org. The more feedback we get the better we can prioritize.