The Mana World (TMW) is a free, open-source 2D Massively Multiplayer Online Role-Playing Game (MMORPG) that has been running continuously since 2004. Built around a top-down, pixel-art fantasy world, it drops you onto the isle of Hurnscald where you explore a large, ever-expanding landscape, fight monsters, complete quests and team up with other players. You earn and craft weapons, armour and equipment, socialise in town, and join guilds — all with a free account and no pay-to-win. The game runs on tmwAthena, a community-maintained open-source server, and the whole project is GPL-licensed so it can never be taken away from its players.
Most free MMOs quietly lean on pay-to-win, ads or gated content. The Mana World does none of that: the server, clients and much of the artwork are open source (GPL), the game has been online for over two decades and still runs a friendly community of active players. It is a genuine, decades-deep MMORPG you can play casually or sink serious time into — on Windows, Linux or macOS — with zero cost and no artificial paywalls.
Linux · Windows · macOS (via the Mana and Manaverse clients; ManaPlus is Linux-only). The open-source server builds from source on Linux.
Free and open source. The tmwa/tmwAthena server is dual-licensed GPL-3.0-or-later and AGPL-3.0-or-later; the game clients are under the GPL; much of the game art and audio is CC BY-SA.
If you like open-world multiplayer RPGs, check out Veloren — a free, open-source multiplayer voxel RPG.