MMO
A massively multiplayer online game (also called MMO and MMOG) is a multiplayer video game which is capable of supporting large numbers of players simultaneously. By necessity, they are played on the Internet. Many games have at least one persistent world, however others just have large numbers of players competing at once in one form or another without any lasting effect to the world at all Read the full article at the Wikipedia.
ARK: Survival Evolved
Ark: Survival Evolved is an action-adventure survival video game.
Genre: MMO, Role-playing 
Unturned
Unturned is a sandbox game in the emerging multiplayer apocalypse survival genre.
Genre: MMO, Adventure 
Imperium
1.5pl0 (12 Sep 2015)What is Imperium?
Imperium is a game of intergalactic exploration, warfare, and economics.
Genre: MMO, Text Based / Visual Novel, Turn-based, Strategy, Role-playing 
Salem
Salem is set in a fantastical New England and offers free form massively multiplayer gameplay in a persistent, mutable and online world.
Genre: Construction and management, MMO, Role-playing 
Strife
by S2 Games
Strife is a competitive online game that features non-stop action and thrilling combat.
Genre: MMO, Role-playing 
Freeminer
0.4.14.8 (16 May 2016)Freeminer is a Minecraft-inspired sandbox game.
Genre: Construction and management, MMO, Role-playing, Simulation 
War Thunder
frequently updated (6 Nov 2014)War Thunder is a next generation MMO combat game dedicated to World War II military aviation, armoured vehicles, and fleets.
Genre: MMO, Simulation, Action 
Blue Saga
frequently updatedA free indie mmorpg currently in development but released in a beta-version, so just download the game and start playing! The game...
Genre: MMO, Adventure, Role-playing 
Evol Online
frequently updated (15 Mar 2016)Escape into the world of Aemil! Join a guild and make new friends that will help you become stronger and a better adventurer! Explore...
Genre: MMO, Role-playing 
Deliantra
3.0 (31 May 2010)Deliantra is a thrilling MMORPG in a retro, pixel art world where magic and monsters abound.
Genre: MMO, Role-playing 
Entropy
by Artplant
Entropy is a sandbox science fiction MMOG with a strong focus on freeform gameplay and large-scale space combat, and is available right...
Genre: MMO, Action 
Warspear Online
by AIGRIND LLC
frequently updated
Create a Hero and battle monsters in a classic 2D MMORPG that has gained popularity with millions of players worldwide! In Arinar,...
Genre: MMO, Adventure, Role-playing