Also https://www.websequencediagrams.com/

and http://www.umlet.com/ (if you need to do more then just sequence diagrams)

And I'd like to add js-sequence-diagrams: http://bramp.github.io/js-sequence-diagrams/

WebSequenceDiagrams also has a Google Docs add-on [1]

[1] https://chrome.google.com/webstore/detail/websequencediagram...

Thanks for that link! That's a great looking tool.