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This is a paid asset, but now you can download Concave Collider Free.
Concave Collider v1.23
The Concave Collider is a component for Unity that enables complex and accurate collision detection and raycasts.
In Unity, mesh to mesh collisions only work if at least one of the meshes is convex.
The Concave Collider helps overcoming this limitation by providing a way to automatically compute a set of colliders that can fit any shape; convex or not. And with just one click!
Includes 5 different sample scenes.
Video | Doc & Help | Forum thread
Requirements:
Unity Pro 3.5/4.x, Unity 5 personal or professional edition. Windows or Mac only. Products build with it will run on any platform.