"Here's a survey of other operating systems that have also been built as open source products, are free to use, and generally come with little to no restrictions over their use. Many of them have taken strong cues from the way Linux has developed, typically in the collections of userland tools that are available. Some owe very little to Linux, apart from the general development philosophy, and are not architecturally related to Linux in any significant way.
"So how useful are these operating systems? In the case of BSD and OpenSolaris, they're already in production deployments all around the world, so there's little to argue with there. HaikuOS, ReactOS, Darwin, and Singularity are entirely another story: they range from being minimally useful on the desktop to only useful as programming research projects."
