ZV Parent Master
ZV Parent Master is an animation tool that helps you to animate objects in mutual contact or interaction with ease.
Classic example: a kid grabbing a ball and playing with it. In this situation you need the ability to constrain the ball to his hands and also to animate the ball whenever he lets it go.
If you rely just on the simple parent constraint, sooner or later you'll realize that this simple task may become a tough job. ZVPM takes care of all the frustrating tasks for you.
ZVPM is a tool that has been around for several years. It's the tool of choice for many big cg studios and animation schools. Over the years I've been improving it, adding features and listening to animator feedbacks. At this point ZVPM has become quite a solid tool. It's free and it will always be. Give it a try! :)
For more details:
Available on Windows, Mac OSX, and Linux