The best open source alternative to Unraid is TrueNAS. If that doesn't suit you, our users have ranked more than 25 alternatives to Unraid and 12 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Unraid are openmediavault, OpenZFS, Rockstor and XigmaNAS.
TrueNAS (formerly FreeNAS) is an operating system that can be installed on virtually any hardware platform to share data over a network. TrueNAS is the simplest way to create a centralized and easily accessible place for your data.
Does not have built in functionality of virtual machines and "Docker" apps.
openmediavault is the next generation network attached storage (NAS) solution based on Debian Linux. It contains services like SSH, (S)FTP, SMB/CIFS, DAAP media server, RSync, BitTorrent client and many more. It can be enhanced via plugins.
OpenMediaVault lacks support for VM orchestration, among probably other things.
OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.
Harvester is the next generation of open-source hyperconverged infrastructure solutions designed for modern cloud-native environments.
(formerly XenServer) Citrix Hypervisor is a leading virtualization management platform optimized for application, desktop and server virtualization infrastructures.
It uses openSUSE Leap as a base with the Btrfs advanced file system. EasyNAS is managed through a web-based interface and offers such features as on-line growing of file systems, snapshots and copy-on-write.
PetaSAN is an open source Scale-Out SAN solution offering massive scalability and performance. PetaSAN uses modern cloud based technologies to provide the elasticity and agility to scale up the storage cluster simply by adding more nodes.
Greyhole is an application that uses Samba to create a storage pool of all your available hard drives (whatever their size, however they're connected), and allows you to create redundant copies of the files you store, in order to prevent data loss when part of your hardware...
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Since the previous comment, FreeNAS, now called TrueNAS has added support for VMs and Docker apps (though to setup a Docker environment, you must do so manually.) It has support for a plethora of things that competitors such as unraid does, such as plugins (for instance Plex, Nextcloud, Jenkins, and many others), ZFS, NAS, SMB, iSCSI, etc., and a very nice web GUI.