GeneralSync

GeneralSync provides shared calendars and address books that can be accessed from Thunderbird, Outlook and most Android apps – without storing anything in the cloud or on servers. Instead of relying on the Internet, data is transferred automatically when two or more devices are connected to the same local network (LAN, Wi-Fi, VPN, ...). In many cases, this enables a similar level of comfort to cloud services, without having to trust a provider or requiring devices to be online. For businesses under GDPR or similar privacy protection laws, this can also reduce the amount of legal paperwork involved as no third party gets access to the synced data.

Besides regular two-way synchronization, GeneralSync also offers to send restricted copies of your data to family members or coworkers. It is possible to work on the same content simultaneously from multiple devices. If conflicting changes are made on multiple devices, users can decide which version to keep at a later time. And as all data is stored on your own devices, there's no limit on the number of calendars, address books to sync or the number of devices to sync with.

GeneralSync is currently free to use, though the developer asks for voluntary contributions to keep the project alive.