Node2Node

Security: This module is considered insecure by Drupal security team and all of it's releases have been unpublished. You are strongly encouraged to disable the module on your site.

Node2Node allows you to create relationships between nodes. The main features are:
- Allowed relationships are defined in the admin screen (ie node type A can be a parent of Node type B)
- Relationships are Parent/Child
- Relationships are many-to-many
- Relationships can have a defined sequence, ie the child nodes of a parent will be listed in a user-defined order
- Relationships can be set up by end users (with editing rights)
- Relationships can be named (ie Node 1 Uses Node 2)

Comparison
Here's a brief comparison with some of the other node relationship modules, of which there are several. Here I am comparing:
- Node2Node (N2N)
- Node Hierarchy (NH)
- Node Relativity (NRel)

Features:
- Defining relationships: N2N lets you specify a relationship between specific node types. NH lets you says that a node type can be a parent (of anything that can be a child). NRel is also very specific about node types.
- Cardinality (is that the right word?): N2N allows many-many. NH allows 1-many. NRel is even more specific, letting you specify 1:1, 1-many, many-many etc
- Sequence: N2N and NH both allow to you to put the child nodes in a specific order. NRel does not seem to specify order.
- Display: N2N displays the parent and child nodes on the View screen. NH puts the parent node in the breadcrumb and the child nodes in a separate tab. (Not sure about NRel)
- Edit: N2N integrates the editing of relationships into the main Edit screen. NH uses a separate tab. (Have to say I'm still working on this and may add other ways of creating relationships later.) (Not sure about NRel.)
- Named relationships: N2N relationships are named (Parent "owns" Child, Child "belongs to" Parent) - you can configure the names for each relationship. Don't think the others have this feature.