OpenSync is a free and open-source app developed by Deepen Dhulla for Android devices that allows users to sync their calendars and contacts using CalDAV/CardDAV protocols. This app integrates natively with the Android calendar and contacts apps, making it easy to use for anyone familiar with the default Android apps.

OpenSync is a fork of the open-source project DavDroid and has been tested with Nextcloud Server. The app's source code is available on GitHub and on Deepen Dhulla's website.

Overall, OpenSync is a great option for those who need to sync their calendars and contacts and prefer open-source software. It's easy to use and integrates well with default Android apps, making it a convenient choice for Android users. Keep in mind that OpenSync is licensed under the GPLv3 License.