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`mscgen` [1] was a popular one back in the day. I think `graphviz` can do it too. Since sequence diagrams are part of UML, most UML tools can do those.

[1] https://en.wikipedia.org/wiki/MscGen

`mscgen' is still my go-to tool for message sequence charts

I tend to like https://www.draw.io/ for that sort of thing (there is a chrome app if you want to run offline).

LucidChart (https://www.lucidchart.com/) is better than draw.io IMO

Looks like https://www.websequencediagrams.com/

Close! I started with that tool, but moved to the FOSS version: https://bramp.github.io/js-sequence-diagrams/