Tilt Brush, a popular VR app that lets users paint in a 3D space. Originally from Skillman & Hackett, it was later acquired by Google and now they've open sourced it.
In an announcement post, Google mentioned it is no longer actively developed and so they are putting it out into the open fully into users hands and so it's now on GitHub under the Apache license. A few systems did get adjusted for the open source release due to licensing but nothing major.
While it doesn't officially support Linux, Google are clearly no longer updating it themselves and so now anyone can come along and fork it and if they wish, provide Linux builds and support for it elsewhere. This is why open source is important, as Tilt Brush can now live on forever and on new platforms. Google are inviting this too, clearly mentioning that Tilt Brush itself is a trademarked name, telling people to change it if they do plan to use the source and clone it.
Do note that the project itself is open source but it was built with Unity, so it still needs the proprietary Unity game engine to work with it directly.
See the original trailer below, nice to see it's not being dumped into a graveyard: