The References project contains straight ports of the node_reference and user_reference modules to the Drupal 7 API.
For a full description of the module, visit the project page: http://drupal.org/project/references
No special requirements
Install as you would normally install a contributed Drupal module. See: https://drupal.org/documentation/install/modules-themes/modules-7 for further information.
It is not envisioned as a final solution, but as a way to actually deploy Drupal 7 from release day on sites using node and user references much as on Drupal 6, until a native entity relationships Drupal 7 module becomes a usable alternative.
As of 2010-11-30, is looks like a candidate for that usable alternative might someday be project Relation: http://drupal.org/project/relation
In short: use these modules now, but be ready to migrate to a different entity referencing solution during the D7 life cycle.
Current maintainers: