samba

Samba is the open source SMB/CIFS server that lets Linux and Mac machines share files with Windows clients (and vice versa). It also runs as an Active Directory domain controller, which is the bigger story for anyone trying to host Windows infrastructure without paying for Windows Server. GPLv3, mature, and shipped in basically every Linux distribution.

Self-hosting is well-trodden ground. Most distros have working defaults, and the community has documented every combination of share permissions, ACLs, and AD trust relationships you can dream up. The real work is the Active Directory side: joining domains, replicating users, and handling Group Policy takes Windows administration knowledge, not Linux know-how.

Solo and small teams: free and the right answer for shared file storage on a home server or small office. Mid-sized teams with mixed Windows and Linux fleets: free for file sharing and a real cost saver if you replace a Windows Server domain controller. Large teams: most still pair Samba with one Microsoft AD instance, but the option to run pure Samba is there.

The catch: SMB has a long history of CVEs. Patch promptly, lock down public exposure, and watch the security mailing list. Free does not mean set and forget.