WordPress.org

This plugin allow to build relation between 2 custom types (posts, page, custom), very useful for manage related content on CMS type website.

A few example use cases:

• manually lists of related posts
• post series
• rented houses connected to agency
• etc.

Relationships are created from a settings page from the administration console.

This plugin can not do relationships with users, you should test this excellent plugin made by scribu http://wordpress.org/extend/plugins/posts-to-posts/