What tag should be used for Massively Multiplayer Online Games?
Options that come to mind:
mmo
Most common? Combined with rpg for games that are both.
mmorpg
Also common. In contrast to tagging both mmo and rpg.
massively-multiplayer
More verbose suggestion. It has been pointed out that although recognisable and in the spirit of minimising abbreviations, it is technically incorrect - as one could have a massively-multiplayer game that was not online.
mmog
More correct. [Removed as the g is basically implied.]