The best free alternative to Spine is Synfig Studio, which is also Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Spine and eight of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Spine are Inochi Creator, enve, AnimeEffects and Live2D Cubism.
Open-source 2D software for creating animations with vector and bitmap artwork, advanced bone rigging, tweening, timeline editing, onion skinning, multi-layer effects, audio sync, and multi-format rendering on Windows, Linux, and macOS.
you can't get it into games and it isn't a runtime
Synfig isn't bone-based 2D animation
Inochi creator is a framework for realtime 2D puppet animation. On this site you can find the reference implementation as well as the official tooling for creating and using Inochi2D puppets for VTubing.
Enve is a free and open-source 2D animation app. It can create both vector and raster animations and even use sound and video files. It has been created with flexibility and expandability in mind.
AnimeEffects is a 2D keyframe animation tool based on deformation of polygon meshes. You can easily operate, quickly shape your idea, and remake at any time if you have changed your mind.
Live2D is the world's first drawing technology to enable 3D rendering of 2D images. This technology supports a variety of portable consoles and smartphones, and Live2D is already being utilized for games that take advantage of the unique characteristics of hand drawn artwork.
Trangram is a free one-stop platform to create, animate and share motion graphics.
E-mote is an engine which can create smooth and stereoscopic animation in a few steps from the character's 2D illustration. You can easily and dynamically move the expressions, orientations, limbs of standing pictures characters and make it look 3D.
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You can't make game animations in Synfig Studio, only animation clips and animation movies