Specifications

iCalendar (RFC 5545) specifies a standard method for representing and exchanging calendaring and scheduling information. This RFC supercedes the original iCalendar specification , RFC 2445. Original text version, iCalendar.org's HTML version

New Properties for iCalendar (RFC 7986) defines a set of new properties for iCalendar data and extends the use of some existing properties to the entire iCalendar object. Original text version, iCalendar.org's HTML version

iCalendar Venue (Draft) specifies a standard method for representing venues beyond what the iCalendar specification provides.  Original text version, iCalendar.org's HTML version

CalDAV (RFC 4791) defines extensions to the Web Distributed Authoring and Versioning (WebDAV) protocol for use with calendar and scheduling information. Original text version, iCalendar.org's HTML version

CalDAV Scheduling Extensions (RFC 6638) defines scheduling extensions to the CalDAV specification (RFC 4791).  Original text version, iCalendar.org's HTML version

Validation Tools

Validation tools help developers find errors in their feeds and to ensure compliance with the iCalendar standard.

http://icalendar.org/validator.html - an iCalendar validator written by Dan Cogliano.

http://severinghaus.org/projects/icv/ - an iCalendar validator written by Ben Fortuna.

Programming Libraries

Programming libraries can help reduce development time building applications featuring iCalendar support.  This list is informational only and does not imply any recommendation on these libraries.

C/C++ - Libical http://libical.github.io/libical/

Java - iCal4j https://github.com/ical4j/ical4j/wiki

.NET - iCal.NET https://github.com/rianjs/ical.net

PHP -

Zap Calendar Library http://iCalendar.org/php-library.html
iCalcreator http://kigkonsult.se/iCalcreator/index.php

Python - iCalendar package https://pypi.python.org/pypi/icalendar

Ruby - icalendar http://icalendar.rubyforge.org/

Calendars

iCalShare.com - A popular site listing thousands of calendars

National Public Holidays - An extensive list of national and state public holidays in icalendar format.

WebCal.fi - A popular site listing calendars for holidays, science, sports and more.

Organizations

CalConnect - A calendaring and scheduling consortium

Internet Engineering Task Force (IETF) - The organization responsible for publishing the iCalendar RFC and other standard protocols

Other

Calendar Swamp - A blog about calendar issues and interoperability calendarswamp.blogspot.com