OpenToonz
Opentoonz is a free and open-source 2D production software tool developed by Dwango in Japan. Check out the animated shorts.
The software has been part of the process of almost all of works of Studio Ghibli. After “Arrietty/The Secret World of Arrietty”, it was customized inside the company, and evolved to become more usable according to its production style.
Official Website:
https://opentoonz.github.io/e/