Creating new nodes from CCK nodereference fields
I can find at least four modules that do this in various ways:
http://drupal.org/project/addnode
http://drupal.org/project/popups_reference
http://drupal.org/project/add_n_reference
http://drupal.org/project/noderefcreate
Older modules that allow this sort of functionality include image_attach and audio_attach (part of http://drupal.org/project/image and audio module respectively). These include extra fields (title and image file, for instance), so for an eventual conversion of these to a generic system, an API needs to lets a module specify which fields the user fills in,
For Drupal 6, there are already some abstractions that work with any node type.
http://drupal.org/project/noderelationships lets you pick existing nodes and also create new nodes of the types allowed by the node reference field.
http://drupal.org/project/nodereference_explorer only supports picking existing nodes.
http://drupal.org/project/nodereference_views_select
:EDIT by Discipolo: I found another - http://drupal.org/node/371518 - Autocreate Node Reference (doesnt use regular nodereference field but defines new one. has fieldformatters for displaying title, teaser and full node)
though it would be nice to have a simple 1-1 parent/child creation like http://drupal.org/node/251526
brandnew http://drupal.org/project/nodeincck
:ENDEDIT
This module supports a specific type of workflow and cardinality, and it's designed for accessibility. It hasn't been released yet.