Nextcloud

Nextcloud Server is a self-hostable platform for storing, syncing, and sharing files, contacts, calendars, and more - positioned as a private, self-controlled alternative to consumer cloud storage and groupware services. Core functionality is extended through an app ecosystem (available from the built-in App Store) covering things like Calendar, Contacts, Mail, and Talk (video/audio chat), so a base install can grow into a broader collaboration suite.

Security features include built-in encryption options, two-factor authentication, and an active HackerOne bug bounty program. You can get Nextcloud several ways: sign up with a hosting provider, install it yourself on your own hardware, buy a device with it preinstalled, or use a dedicated Nextcloud service provider; a separate Nextcloud Enterprise offering targets business, public sector, and education customers.

It's a fit for individuals, organizations, or public-sector teams that want to keep their files and collaboration data on infrastructure they control.