CardDAV-Sync is a CardDAV client for Android to synchronize contacts. Due to its implementation as sync adapter it integrates seamlessly with the default contacts app.
(Also check out CalDAV-Sync)
Important note for Android 4.1 and Samsung users: To prevent the removal of your accounts on reboot install this app:
https://play.google.com/store/apps/details?id=org.dmfs.jb.workaround.carddavsync
At present, Samsung users still need this workaround, even on recent devices....
