DAVdroid is the only all-in-one synchronization solution for your contacts (CardDAV), calendars and tasks (CalDAV). The app is easy to set up and integrates perfectly with your favorite calendar/contacts app (including default apps). It can also be used separately, if you either have CalDAV, CardDAV or only Tasks.

Compatible with almost every CalDAV/CardDAV servers and services, including OwnCloud, iCloud and OS X Server!

After installing, you can add a DAVdroid account from within the app. See https://davdroid.bitfire.at/configuration/ for help. For more information, please see the FAQ: https://davdroid.bitfire.at/faq/ and our forums: https://davdroid.bitfire.at/forums/

Key features:

⊛ Sync your DAV calendars (CalDAV) and address-books (CardDAV) and Tasks in a single app
⊛ Two-way synchronization (server ↔ client)
⊛ Native Android 4+ implementation
⊛ Flawless integration with your device and favorite apps
⊛ Easy setup (resource auto-detection)
⊛ Fast algorithms for high performance (CTag/ETag support)
⊛ Optimized for phones and tablets
⊛ Super secure and we respect your privacy!
⊛ DAVdroid is completely open source!
⊛ Available Languages: Englisch, German, Czech, Chinese, Hungarian, Catalan, Spanish, Portuguese, Russian, Serbian, Dutch, Francais

Successfully tested with:

⊛ AOL
⊛ Apple's Calendar and Contacts Server
⊛ Apple iCloud
⊛ Baïkal
⊛ contactzilla
⊛ CozyCloud
⊛ DAViCal
⊛ eGroupware
⊛ fastmail
⊛ fruux
⊛ gmx
⊛ Horde
⊛ Mac OS X Server
⊛ mailbox.org
⊛ mail.de
⊛ Kerio Connect
⊛ KolabNow
⊛ OwnCloud
⊛ posteo.de
⊛ Radicale
⊛ SOGo
⊛ Synology DSM
⊛ web.de
⊛ Yahoo (calendars only)
⊛ Yandex
⊛ Zimbra Server (Community Edition)

Screenshots: