Joda Time seeks to reimplement date handling from scratch in Java. Both Date and Calendar in the core Java libraries have serious flaws, and this project seeks to address this. This is the most active project.
Joda Primitives provides a set of interfaces and implementations for collections designed for primitive types. These can be much more efficient than wrapping each primitive in an object.
Joda Beans seeks to provide the next generation of handling for beans. It provides a standard interface for beans to implement that allows each property on a bean to become a first class object. This enables programs to work in a whole new way.