Unity added 2D support with their 4.3 release of the engine. The 2D package comes with options for both sprite sheet animations and bone based animations with the added support of
GAF:Flash to Unity
GAF: Flash to Unity allows using Flash animations with Unity.
Smooth Moves is a bone animation plugin for Unity 3D that supported 2D games with bone animation before the 4.3 release.
Puppet2D integrates with Unity’s animation system and adds mesh deformation to create bendable joints for skeletal based animation.