Included with Mac OS X 10.6
Desktop client for Mac OS X that supports CardDAV. Also provides CardDAV-based auto-complete to other apps via built-in APIs.
Included with iOS 4
Client for iPhone/iPod Touch that supports CardDAV.
Included with Atmail 6 Product PHP/Zend Email Server with jQuery browser based WebClient
Supports CardDAV to list, view, edit contacts directly on the server. Server contacts and CalDAV calendar syncing with ActiveSync to iOS and downloadable Outlook Add-in. Free Trial Available.
Commercial/Outlook plug-in
The WebDAV Collaborator is a powerful plug-in for synchronizing your CalDAV calendar and/or CardDAV contact data to Outlook.
Android sync adapter
CardDAV-Sync is a sync adapter for Android devices that synchronizes contacts via CardDAV. See https://market.android.com/details?id=org.dmfs.carddav.sync for ratings.
Available in free and purchased editions
Open Source CardDAV client
Desktop open source client (Javascript+jQuery) for CardDAV.
Android sync adapter
ContactSync is an Android app which syncs your Android contacts with FTP, HTTP, WebDAV, CardDAV servers and plain vcard files. It also provides a lot of additional features.
Free/Windows
Desktop client for Windows (.NET) supports CardDAV.
Contact Manager
Emacs BBDB3 is a contact manager supporting CardDAV.
E-mail program
Evolution is an e-mail program which supports CardDAV.
Open Source/Cross-platform
Desktop client that supports CardDAV.
Client and Server
OwnCloud is a web client and a server supporting CardDAV.
Open Source CardDAV client
PHP CardDAV Client.
Python CardDAV client and library
pyCardDAV is a CLI CardDAV client (for use with mutt etc.) and also contains a reusable CardDAV module.
Open Source CardDAV plug-in
Open source CardDAV plug-in for Roundcube Webmail.
Free/Thunderbird Plugin
Plugin for Thunderbird address books which allows CardDAV subscriptions.