ownCloud

A file sync and sharing platform that turns your own storage into a private cloud for files, folders and documents. Desktop and mobile clients keep local copies in sync over WebDAV, spaces act as project folders with their own members, and links can carry passwords and expiry dates.

It suits teams that need shared drives and external sharing without sending regulated documents to a public cloud.

Deploying it on your own infrastructure keeps every byte in your data centre, avoids per-user subscriptions, and makes backup a matter of copying the data directory.