I want to create a calendar in my application that external users can view.
The CalDAV (basically WebDAV+iCalendar) format seems to be relatively widely supported, altho
I've used http://sourceforge.net/projects/dday-ical/ for a project last year. Works perfectly!
The company IT Hit WebDav System offers
IT Hit WebDAV Server Engine v3 for .Net
(starting at $ 1250)) and IT Hit WebDAV Client .Net
(starting at $350). It works with iOS Calendar, Android CalDAV-Sync, Mac OS X Calendar, Mozilla Lightning, eM Client, Bynari Outlook plugin, Evolution and others.
If what you are looking for is a CalDAV Client then my suggestion is DAVDroid, this is the best CalDAV Client I have never tried. It's a connector for the calendar account of your phone allowing to consume the data from your CalDAV Server from any Calendar App installed on your phone.
Other interesting CalDAV Clients are:
Windows:
If you are looking for a ICalendar library for .NET you can check the one my friend and I build: ICalendar
Any doudt you can ask us!!!