Zimbra

 Zimbra  is an open source email and calendar web application often used by organizations internally, and has the ability to subscribe to ICS feed files generated from hCalendar like the IndieWeb events page.

How to subscribe
After logging into Zimbra: (i.e. from the home screen)
 * Choose the "Calendar" tab
 * in the far left column, click the new calendar icon (little mini green calendar with a plus sign, top right corner of the column header), a "Create New Calendar" dialog box should appear
 * enter "indie web events" into the Name field
 * check the 2nd checkbox:  [x] Synchronize appointments from remote calendar
 * copy this URL:  
 * paste it into the URL field and click  [ OK ]