Unraid is described as 'Flexible OS manages mixed drive storage, runs Docker containers and VMs, supports hardware passthrough, offers user-friendly web interface and parity protection' and is a NAS system in the network & admin category. There are more than 25 alternatives to Unraid for a variety of platforms, including Linux, Self-Hosted, Windows, Mac and BSD apps. The best Unraid alternative is TrueNAS, which is both free and Open Source. Other great apps like Unraid are openmediavault, ZimaOS, OpenZFS and Rockstor.
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.
Most users think TrueNAS is a great Unraid alternative.
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.
Most users think openmediavault is a great Unraid alternative.
OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.
Rockstor is a free and open source NAS (Network Attached Storage) operating system. It’s a software solution and can be installed on commodity hardware or a hypervisor satisfying minimum system requirements.
XigmaNAS (formerly NAS4Free) is an embedded Open Source Storage NAS (Network-Attached Storage) distribution based on FreeBSD. This project is a continuation of TrueNAS 7 series project.
Harvester is the next generation of open-source hyperconverged infrastructure solutions designed for modern cloud-native environments.
Software-defined infrastructure for cyber protection. Streamline IT efforts using built-in management and monitoring tools. Get integrated support for multiple workloads, use cases, storage types and APIs. Maximize productivity, lower operational costs, and enable growth.
Microsoft Hyper-V Server is a free product that delivers enterprise-class virtualization for your datacenter and hybrid cloud.
Western Digital My Cloud is an easy-to-use personal cloud storage device that plugs directly into your Wi-Fi router at home so you can preserve your digital life. Use the My Cloud app on your phone, tablet or computer to access it anywhere without paying a subscription fee.
(formerly XenServer) Citrix Hypervisor is a leading virtualization management platform optimized for application, desktop and server virtualization infrastructures.
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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.