Freemind

Freemind is mindMapping software. It allows you to create one root node and a hierarchical tree of child nodes. It also allows to fold in trees of whatever level, so you cannot see its sub-nodes. This is useful for practicing.

Freemind does not allow more than one root node and it does not allow a mesh structure, only a star structure.

Links

 * Wikipedia's article
 * Freemind project's homepage
 * Wikipedia's list of mindmapping software