The node-reference module (a component of CCK) provides a great way to create a reference one node from another. What is the best way to expose a two-way relationship between the two? Example below:
For instance, in a ticket tracking system with two tickets (nodes) A & B, you could create a node reference from ticket A --> B and mark B as a child. When viewing ticket A it would be trivial to display this information. However, I would also like to display this relationship when viewing ticket B and show ticket A as its parent.
This same requirement would also apply in non-hierarchical situations (tickets C & D) with a reference from C --> D, the relationship should be exposed on ticket D as well as on C.