This is a project I've been working on for over a year. Many alternatives for 2D skeletal aniation are either prohibitively overpriced (Spine, Creature, etc.), outdated (DragonBones), or are tied to a specific game engine (Unity, Godot, etc.).

SkelForm is simpler than its peers, but it has support for the advanced stuff: inverse kinematics, mesh deformation, damping and swaying-based physics. The editor supports PSD importing and, beyond just the armature, exporting spritesheets or videos.

As well as the usual user documentation, its runtime specification and implementation is fully documented separately (albeit mostly with pseudo code): https://skelform.org/dev-docs

The currently supported game engines are moreso just frameworks like Macroquad and Pygame, since those are what I wanted to make games on and are easy to maintain. Someone else is developing a Godot runtime, but I plan to develop the Unity runtime once I can devote my time to learning it.