An open standard for a RISC CPU, developed at UC Berkeley
"RISC-V is a specification of a vocabulary for a computer. The closest analogy to the RISC-V specification is a book that defines words, like a dictionary. A dictionary can’t run programs."
—RISC-V Foundation FAQ
Developed at the University of California, Berkeley] in 2010, this is an open-source instruction set architecture (ISA) with a RISC design philosophy. Since the initial design, it passed through the hands of the RISC-V FOundation, thence to RISC-V International, a non-profit organisation based in Switzerland.
The design is freely available to anyone desirous of using it, under a Creative Commons license or a BSD License. This means that would-be users of the ISA may use it to build a CPU to sell or use as part of another product.
Who is using it?
Historically for the most part, use has been limited to microcontrollers, or in embedded systems. Western Digital use it in their storage devices to optimize performance and reduce costs. SiFive is a company that provides RISC-V-based processors and development tools for various applications and even NVIDIA are Exploring RISC-V for certain aspects of their GPU architecture and development.
It's slowly finding its way into many other applications, notably IOT and 'smart devices' and increasingly, autonomous automotive applications, edge computing applications, in academia as a learning and research tool and increasingly in AI/machine learning.
GNU/Linux has a kernel and toolchain for use with the architecture, and complete consumer RISC-V systems are currently being produced so early adopters and developers can take advantage of it. Most major Linux distros have images for it, to run on bare metal or in virtual machine\virtual machines.
one company, SiFive, is already developing hardware with the ISA, including several complete development boards, and the Framework laptop company is considering a laptop based on a RISC-V cpu. The Beagleboard company is producing a single-board computer akin to the Raspberry Pi ¹ I'm debating getting one just to play with. Look out, RISC-V is coming to change your life even more.
¹ https://www.beagleboard.org/boards/beaglev-fire
$ xclip -o | wc -w