Included with Mac OS X 10.6
Desktop client for Mac OS X that supports CardDAV.
Open Source/Cross-platform
Desktop client that supports CardDAV.
Free/Windows
Desktop client for Windows (.NET) is adding support for CardDAV.
Free/Thunderbird Plugin
Plugin for Thunderbird address books which allows CardDAV subscriptions.
Commercial/Windows
Outlook plugin that integrates CardDAV collections as contact folders.