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.
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 
Wander
frequently updatedWander is a collaborative, non-combat, non-competitive game focused on exploration and joy.
Genre: MMO, Social, Role-playing 
Heroes of Umbra
frequently updatedHeroes of Umbra is a 2D side-scrolling online RPG in-progress that plays similar to side-scrolling RPGs like MapleStory and Castlevania.
Genre: MMO, Platform, Role-playing 
PokeMMO
frequently updatedPokeMMO is a fan-made MMORPG based on the 3rd generation of GameFreak's popular Pokémon video game franchise.
Genre: MMO, Role-playing 
Other Life
by Learner
frequently updated
Other Life is rather cute 'old school' mmorpg in the fantasy genre that started in October last year.
Genre: MMO, Role-playing