Drupal - How do I implement two-way node references?

There are many modules available, your milage may vary:

BackReference (Drupal 6 only)

NodeReferrer (Drupal 7 beta release)

Node Relationships (Drupal 6 only)

Corresponding node references (Drupal 7 stable release)

Corresponding entity references (Drupal 7 dev release)

Content Dependency (Drupal 7 stable release)

Entity Reference Module now works with Corresponding Entity References. This is ideal for all nodes and non node entities.

For Drupal 6, the best option is probably the NodeReferrer module. For Drupal 7, Relation fits the bill but is not yet ready.