The Userbar module provides an easy way for modules to automatically notify end users about any change in their content status.
For example, when users flag a content with the Flag module, they get notified in the Userbar when the flagged content has been updated. Other modules like the Private message module can notify the user when a new private message has arrived.
Userbar is a nice addition to all modules which change their content very often and need to inform the specific user about that. Administrators can configure which roles can access and personalize the Userbar, while Users can select those items for which they would like to get notified.
In addition to that, the module provides two hooks (hook_userbar_info and hook_userbar_view) which all contributing modules can implement to provide the latest status about their content that is owned by them.