The screenshot on the left is the Stone Training mini-game, where you learn how to throw stones at targets. The one on the right is The Arena, where you can fight with a friend.
Plee the Bear is not only a game, it also serves as a support for the Bear Engine, a free C++ 2D-game engine including physics with collision detection, graphics with sprites, animations and bitmap font support, a sound system, a scripting module and a complete set of editors for levels, animations and characters. See the engine's API online.
This engine is game independent, the items of your game will indeed be integrated through plugins. The custom-bear archive is a good start for your project, also is this archive which is the source code of a tutorial to learn how to create an item for the engine.