The Drupal 7 successor to Build modes which will allow administrators to define custom view modes for entities. Custom entities are added to the entity registry via hook_entity_info_alter() so they are available to any code that uses entity_get_info() to provide a list of view modes for an entity. This includes node and user reference fields, Views, etc.
It also ensures consistency for template suggestions for all entity types, so that you can use any of the template patterns, in order of most specific to least specific:
Display Suite - a powerful, but large and complex module for controlling view modes, layout, exportables, fields. You can use Display Suite to define view modes but it better fits the use case of needing the extra functionality as well rather than *just* new view modes.
Development was sponsored by Palantir.net.