This mod adds back a feature that has been missing since 1.3 and the client-server merge. That feature is the ability to be pushed around by other mobs and players.
Walking into a player will move them. If either player is sneaking, the entities will not be effected, but sneaking will not effect a mob.
Landing on another entity will break your fall, but they will get 3x your fall damage. (1.8 only)