List of free massively multiplayer online games

This is a selected list of notable massively multiplayer online games which are free-to-play in some form without ever requiring a subscription or other payment. These are commonly MMORPGs or MMOFPSs, but could be of any genre.
Free play.
These MMOGs provide client software free of charge and allow users to play the game without requiring payment. The games' expenses are typically funded by sponsors or through donations (which have no effect in the game itself).
Free play with advertising.
These MMOGs are free to play. They are funded through advertising, either in-game or through pop-ups.
Free play with micro-transactions.
These MMOGs are free to play, but players may optionally purchase in-game items or currency.
Optional paid subscriptions.
These MMOGs offer optional additional game content through paid subscription, but are otherwise free to play.
Buy to play.
These MMOGs generally require an upfront cost to purchase the game client but are generally free to play without subscription fees. Players may optionally purchase in-game items or currency.