Is there a module that will let me set up site-wide notifications like Stack Overflow's orange one? I'm looking for a Drupal 7 compatible module that will let me set up site-wide notifications similar to Stack Overflow's orange bar (Welcome to this site, read the FAQ etc.). I haven't been able to find one.

Please note that I don't want to display system messages like this, I want something where I or other selected users can set up custom ones.
Check out the list of modules for Displaying Status Messages — some of these might provide a good starting point.