There's currently no rest or html public view of the mail folders so it was really confusing people - you'll have to use CLI to share with the public for an rss feed.
Bug 18513 - Remove "external" and "public" options from the folder share dialog
Under construction:
Bug 26763 - Tasks - Rest HTML interface Bug 26764 - Contacts - REST HTML interface
5.0.5 added:
Bug 22561 - No REST/HTML UI to list folder contents of publicly shared briefcase folder
5.0.1 included the ability to get free busy info by visiting http:// server.domain.com/home/username?fmt=freebusy which will display an aggregate HTML calendar for all the user's free-busy data. (Of course you can always choose to select "exclude this calendar when reporting free/busy times" if you wish.)
In 5.0, we added the ability to go to http:// server.domain.com/home/username/Calendar.html to see an easily readable day/week/month view. (The publisher must share the calendar specifically or publicly in order for you to see it.)