I used Spine a while back. It's such a lovely tool with a great UI, and compatible with Linux.
It's neither free nor open source though so I don't know if this is very suitable here on HN.
In any case, I wrote an exporter for Inkscape since they only had an Illustrator plugin for vector graphics:
AFAIK the team behind both libgdx and Spine are quite good at giving back to the community. After libgdx they quit their day jobs and went full-time as Esoteric Software [3] and did an excellent job following through with Spine's two kickstarters.
I think this is very relevant to HN not only because of the guys' open source contributions but also their entrepreneurial history (success?).
[1] https://github.com/libgdx/libgdx [2] https://github.com/EsotericSoftware/spine-runtimes [3] http://esotericsoftware.com/
(I'm not affiliated with ES but was a backer of Spine and am a user of libgdx)