RISC OS is described as 'Risc OS is an open-source OS specifically designed to run on the ARM chipset' and is an app in the OS & Utilities category. There are more than 100 alternatives to RISC OS for a variety of platforms, including Linux, BSD, Self-Hosted solutions, Windows and Mac. The best alternative is Ubuntu, which is both free and Open Source. Other great apps like RISC OS are Manjaro Linux (Free, Open Source), Arch Linux (Free, Open Source), Windows 10 (Paid) and Fedora (Free, Open Source).
FreeBSD is an advanced operating system for x86 compatible (including Pentium® and Athlon™), amd64 compatible (including Opteron™, Athlon™64, and EM64T), ARM, IA-64, PowerPC, PC-98 and UltraSPARC® architectures.
Ubuntu Mate is a stable, easy-to-use operating system with a configurable desktop environment. Ideal for those who want the most out of their desktops, laptops and netbooks and prefer a traditional desktop metaphor.
Red Hat is the leader in development, deployment, and management of Linux and open source solutions for Internet infrastructure - ranging from embedded devices to secure Web servers. Red Hat was founded in 1994 by visionary entrepreneurs Bob Young and Marc Ewing.