Threema apps are now open source under the AGPLv3
threema.ch
threema.ch
One single large commit. From the README: "There will be one commit per released version."
They did leave Github pull requests open, for now, but I guess that is just a configuration mishap. README: "For the time being, we do not accept contributions via GitHub. If you want to make a contribution, first make sure that you have signed the CLA. You can then submit patches over email by following the instructions at threema.ch/open-source/contributions/."
Threema has by far the most responsive UI of all messengers on Android. It does not require phone numbers and has a sane security model. Unfortunately like most others it doesn't support multiple profiles, and it doesn't have message expiration, something that I really miss. Still my favorite messenger!