Replicant is a fully free Android distribution running on several devices, a free software mobile operating system putting the emphasis on freedom and privacy/security. It is based on LineageOS and replaces or avoids every proprietary component of the system, such as user-space programs and libraries as well as firmwares.
Replicant aims to be an ethical system: it does not ship nor recommend the use of non-free software.
Many different devices are supported by Replicant, to an extent specific to each device. Basic features such as graphics, sound and telephony (if applicable) are supposed to be working for each device (if not, developers are confident they will work soon).
Replicant is a community-led effort, currently driven by few developers. Not every piece of software in Replicant should be expected to be fully-featured nor entirely reliable, even though developers are trying their best.
Comments and Reviews
Replicant does what it does well. It supports devices from about a decade ago, maintains them so people can still use them comfortably, and does all this without proprietary blobs.
Most people will find more use in DivestOS, though. It supports a great deal of newer phones, which is more than the zero that Replicant supports. ;)
A great concept to be sure, but when there are less than ten supported phones, the newest of which is from 2012, it's just not a good alternative to anything. It seems as though this project has been discontinued, as there is no active support for devices past the Samsung Galaxy S3, and seemingly no plans to develop support for any more. LineageOS, which installs without Google or Google services, fills much of the void that Replicant was meant to fill.