Plastic Animation Paper
Just stumbled across this piece of software.
It's not OSS but they have Linux version which is trend I support all the way...
Haven't tried it but some folks here might find it interesting.
Basically, it is software for: "Easy and efficient 2D software
for drawing traditional animation the digital way!" as stated on their website.
Check it out.
P.S I am not in ant way affiliated with this company...just thought people should know about software, commercial or otherwise which is available.