MakeHuman is a free open-source software that allows you to create and pose 3D human characters. Useful for pose reference.
MakeHuman (FREE)
Pencil 2D (FREE)
Pencil 2D – An animation software for vector and bitmap graphics. It is free, multi-platform and open source
Synfig Studio (FREE)
Synfig Studio – A free open-source 2D animation software
Animator's Resource Kit 