Recs.
Updated
Specs
Pros
Pro Integration libraries support most engines
Spine provides integrated modules for 14 major engines including GameMaker Studio, Cocos2Dx, LOVE, MonoGame, Unity, XNA, Flash, HTML 5, libgdx, Corona, and more. Generic runtime libraries are also available for C, C++, Objective C, C#, JavaScript, Lua and ActionScript 3. Dozens of third party libraries support additional targets.
Pro Simplified UI allows for an easy learning curve
No-nonsense workflow allows you to create your artwork in programs much better suited for the task like Illustrator and Inkscape for example, while Spine itself focuses only on the important task of skeletal animation.
Pro Community-driven feature roadmap
Esoteric Software maintains public Waffle issue trackers to help plan and prioritize feature additions.
Pro Funded via Kickstarter
Spine received resounding support from Kickstarter backers, beating its pledge goal by over 5 times. A second successful Kickstarter added meshes and other important features.
Cons
Con Spine Professional is expensive
Spine Essential includes nearly all features, except IK, weights, and meshes. Spine Professional is expensive, though it does give all future updates for life (Spine is updated very often).
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Flagged Pros + Cons
Pro Runtimes, integration, and third-party support for dozens of engines:
GameMaker Studio, Cocos2Dx, LOVE, MonoGame, Unity, XNA, Flash, HTML 5, libgdx, Corona, and more.
Generic runtimes for:
ActionScript 3
C / C++ / Objective-C
C#
JavaScript
TypeScript
Lua