OpenToonz is a free and open source animation software that has been in existence under a number of different names for almost 30 years. OpenToonz is battle tested in production environments including the infamous Studio Ghibli (Princess Mononoke, Howl’s Moving Castle, etc.) and Rough Draft Studio (Futurama, etc.).
OpenToonz was first released publicly on IRIX systems and distributed by Softimage all the way back in 1993, then called Toonz. In 2016 Toonz was open sourced as OpenToonz, although a premium commercial version Toonz Premium is still produced to this day (link below).
OpenToonz provides features for the entire animation process, from scanning and transforming traditional drawing tools, to raster and vector based drawing tools, to special FX and finally rendering.
OpenToonz is open source under the BSD-3 open source license with the source code hosted on GitHub.
You can learn more about OpenToonz, including a quick tutorial on creating a basic animation, in the video below.