Blendings / Inbetweens 
Inbetween World Matrix
Calculates the value between the next and previous keyframes, but in world space values. Useful to lock sliding legs in space, or for creating an offset in world space.
Inbetween Keyframes
Adds a keyframe and calculates the value between the next and previous keyframes, but for each layer or an nla track separatly. This slider is taken from animation layers tools.
Blend Mirror
Blends the current pose to the mirrored pose. Useful when mirroring a pose but doesn't want to have a complete symmetry.