After a quick search on github.com, the following projects came to my attention:
Out of the three, OpenProject appears as the most mature, good looking and reliable solution.
Rather free online tools:
An awkward choice would be Taskwarrior, which is mainly used in command line:
An interesting and simple comparison between kanban and scrum: https://www.youtube.com/watch?v=9Jgu1BlTlSc
And a related github repository, for invoicing application: