Commercial OS X Server, also Open source/cross-platform
Implementation of CalDAV Access and Scheduling. Available as part of the OS X Server software. Also open source. Python-based.
Open source/cross-platform
Implementation of CalDAV Access and Scheduling. Java-based. Includes a Web UI.
Open Source
Java-based server that includes support for CalDAV.
Open Source/PHP/PostgreSQL
Implementation of CalDAV Access and Scheduling with web-based administration.
Open Source/Java
DavMail is a POP/IMAP/SMTP/Caldav/LDAP exchange gateway allowing users to use any mail/calendar client with an Exchange server through Outlook Web Access.
Open Source/Perl/CGI
Dingo is a CalDAV compatible server using MySQL, written in Perl as a CGI script.
Free
EGroupware is an enterprise ready groupware server that includes CalDAV support.
Commercial
Implementation of CalDAV Access and Scheduling as part of unified messaging server.
Open source/cross-platform
Apache plugin implementing CalDAV Access.
Commercial
Implementation of CalDAV Access and Scheduling.
Open source / Cross platform
Radcale is an open source cross-platform CalDAV and CardDAV server written in Python. It requires few software dependencies and is pre-configured to work out-of-the-box.
Open Source
SabreDAV is a free WebDAV server and includes support for CalDAV.
Open Source
ScalableOGo (SOGo) is a free groupware server and includes support for CalDAV.
Commercial/Open Source
Implementation of CalDAV Access and Scheduling.
Commercial/Linux
Synovel Collabsuite is an open source technology based mail and collaboration software. It provides comprehensive Linux mail server with calendaring (CalDAV) and instant messaging functionality for enterprise.
Commercial
Implementation of CalDAV Access and Scheduling. Formerly Sun Java Calendar Server.
Commercial/Open Source
Implementation of CalDAV Access and Scheduling.
Open Source
ownCloud is an open source, simple cloud server, providing WebDAV, CalDAV, CardDAV, etc. It's written in PHP and can use SQLite or MySQL databases. So it runs under Windows, Linux, etc.
Open Source
Baïkal is an open-source, lightweight PHP+SQLite CALDAV+CARDDAV server based on SabreDAV.
Commercial Collaboration Suite
BCS (Bynari Collaboration Suite) is a complete groupware/messaging suite which includes CalDAV support.
Shareware/Trial Period
CalDAV and CardDAV support.
Commercial CalDAV Server
Daylite 4 is a commercial CalDAV server
GPL/Joomla
DPCalendar allows CalDAV access to its event system on your Joomla site.