Open source/Java
CalDAV4j is a protocol library that extends the Slide project's WebDAV client library (which itself is an extension of the Apache's HttpClient library) to allow high level manipulation of CalDAV calendar collections as well as lower level CalDAV protocol interactions.
Open source/Python
A protocol library for CalDAV with an interactive shell.
Open source/Python
This project is a CalDAV (RFC4791) client library for Python.
Open source/Java
iCal4j is a Java library used to read and write iCalendar data streams as defined in RFC2445.
Open source/C
An open source library that is the basis for the CalDAV plugin currently under development for Kontact.
Open source/C
An open source reference implementation of the iCalendar data type and serialization format.
Java library
Milton is a java library for WebDAV, CalDAV and CardDAV server implementations.
Open source/C++/Python/Java
The library used by the Mulberry client for its iCalendar and iTIP handling. Also includes Java and Python ports with similar APIs to the C++ one.
Open source/Ruby
Ruby library which provides parsing, generation, and usage of RFC 2445 (iCalendar) data, including support for enumerating occurrences of recurring components and time zone conversions.
Open source/Objective-C
The SOPE iCal package (sope-ical) contains a SaxObjC driver for processing iCalendar files using libical (iCalSaxDriver) and a library which contains model classes for iCalendar/vCard objects.
Open source/Python
A Python library used to read and write iCalendar data streams as defined in RFC2445. Also does vCards.
Open source/Python
zanshin is a library for collaboration over HTTP, WebDAV and CalDAV. Its primary client is the Chandler SharingProject: for project management-related information, see WebdavService.