This is a small module that enables the automatic population of the menu title field following changes made to the node title in an add or edit form. It is designed to save time with data entry. The feature can be enabled or disabled on a per-content type basis, and allows overriding the menu title for custom titles when needed.
Note: A similar sort of functionality, is already present in Drupal 7.0, as discussed by dman.
There are a number of other modules that provide similar functionality via different means. Probably the best one that merges all the functionality together is the Automenu module.