P2P UI is a WordPress plugin designed to be used in conjuction with Posts 2 Posts, a plugin by scribu. Posts 2 Posts URL: http://wordpress.org/extend/plugins/posts-to-posts/
The basic idea is a post-type of connection, which holds information about posts connections. These connections are then registered at init.
Additionally, a post-type of data-type is created. This is used in a similar manner to create new post-types.
This is a very simple, somewhat hacky implementation of this idea. If you think the bones are good and want to contribute here, please fork me. If you'd like to steal the idea and start from scratch, please do so (and let me know).