Here are the criteria to be on this list:
¹ If only the client or server is available
under one of the stated licenses, it will be denoted in the link.
Graphics and dependencies may be licensed otherwise within the given ruleset of
compatibility for the GPL license chosen. Generally, graphics can be proprietary
but the dependencies cannot but may use a different compatible license.
² This will be determined by checking to see if anyone is playing on the game's servers OR if a contribution has been made to the project's repositories/forums within the past year of present day.
³ This basically means that network-play games will not be considered MMO unless there is project/developer-owned site that monitors and publicly displays the status of active servers/networks that players may join. A minimum of one server/network must be monitored at all times by the site in question.
Here is the current list: