In case you somehow haven't already heard, the Principia source code has finally been released after 6 years.
On the 5th of August, Emil (or sdac, which you might know as the founder of Bithack AB) joined the Principia Discord Server and finally came back to the community. He promised us to open source the game and further its development as an open source project, and kept his promise, later releasing the source code about a day later. The source repository is available at:
https://github.com/Bithack/principia
For Arch Linux users I've already made an AUR package tracking and building the latest master branch, but for other Linux users or Windows users you will either have to compile from the master branch or wait until there are new officially available binaries. The Android version is going to be updated to work on newer versions of the Android OS and will be made available on Google Play for free. For the time being however, principia-web will be hosting the legacy proprietary 1.5.1 versions with mods applied before these are released.
As for community sites, principia-web will stay and is going to become the community site going forward for the open source Principia project, although the community site will be configurable to allow for pointing to somewhere completely different. It will see many updates in the future thanks to the source code release allowing for things like screenshotting to be done automatically without intervention, reimplementation of leaderboards and various other things such as bringing back puzzles and level packages.
You will also be able to freely play levels from separate sites hosting a compatible server software, including an archive of the official community site which works on recent Git versions of the open source Principia.
There are still a lot of things to be done and a lot of work ahead in preparation for the first officially open source Principia version, but this is going to be an exciting time going forward. If you'd like to get involved in a semi-official capacity then Emil has set up a form to do so (form is no longer active).
And if you haven't already, please join Principia Discord Server as it is the place where most development activity happens now (for better or for worse... can be bridged to IRC or Matrix if there is demand). Also check out @bithack on Twitter and make sure to star the Principia repository on GitHub!