Recommended and mentioned products
Mudlet is a freshly-minted MUD client, designed to take mudding to a new level.
Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux.
Xonotic, one of many FOSS arena shooters, the continuation of Nexuiz, with good graphics and fun gameplay.
Join "The Project" to restore humanity's synaptic-link technology after nuclear war...
Warzone 2100, a post-nuclear 3D RTS that is a lost of fun, originally a commercial game ~20 years ago but it has been fully FOSS since 2008.
A multiplayer voxel RPG. Inspired in games such as Cube World, Breath of the Wild, Dwarf Fortress and Minecraft.
Veloren, a 3D exploration / survival game heavily inspired by cube world (but it's much better).
Flare (Free Libre Action Roleplaying Engine) is a simple game engine built to handle a very...