Parent

Parent is a fully dynamic tool for parenting any object, light, or bone to any other. You can switch parents at any time to allow "hand-offs" of objects from one character to another. You can use it to attach clothes and jewelry to LightWave bones so they stay attached when your character is animated by the bones.

Parent is useful for more than just character animation. If you want to have an attack helicopter launch a missile, you have a dilemma. You want the missile to be attached to the helicopter before its launched. The helicopter may be weaving through a canyon and it would be very painful to keyframe the missile taking exactly the right path to appear to be still connected to the helicopter. But you can't parent the missile to the helicopter, because you want it to launch and proceed on its own afterwards. What you want is dynamic parenting, where the missile is attached to the helicopter until the launch, but free to be manually keyframed afterwards. This is easily and elegantly done with the Parent plugin.

Parent has many more features such as divorcing channels (allowing you to make cockpit attitude indicators and objects that will stay upright even if their parent rotates.) It allows cycled parenting, which allows you to repeat a sequence of parents.