Recast Navigation

Recast takes raw level polygons and voxelizes them: the geometry is rasterized into a heightfield, walkable surfaces are grown into regions, and those regions are simplified into convex polygons. Out comes a navmesh without a designer having to draw one by hand. Its sibling library Detour is the runtime half — A* search, path following, and a crowd module that keeps many agents from walking through each other on the mesh.

Mikko Mononen released it around 2009 under the zlib license, and it became the de facto standard: Unity's older navigation and a long line of game engines build on it or imitate it. That navmesh generation feels like a single button press in modern editors is largely Recast's doing.