Starting with 5.0 or so (January 2008), Zimbra has supported its own simple, read-only HTML view of calendars. URLs are like
https://mail.example.com/home/claude.../Calendar.html
Starting with 5.0.11 or so (October 2008), you can right-click on the name of the calendar in the upper left pane and choose "Launch in a separate window." Access control is as usual.