MUDs (Multi-User Dungeons) may not be as popular as MMORPGs, but the genre still thrives. Most MUDs are text-based, and usually offer more depth than their MMO cousins. Furthermore, a large majority of MUDs are free. 

Free Online RPGs 
Free multiplayer online-only games with RPG elements. 

Free MMORPGs 
Check out free multiplayer games on a massive scale. 

MUDs

Realms of Despair
Graphics: No
Download: No
Free or Pay: Free
The development grounds for the SMAUG code and remains one of the most stable and popular MUDs. RoD is newbie-friendly and has clans, guilds, orders, and councils. 

Wheel of Time MUD
Graphics: No
Download: No
Free or Pay: Free
Text-based RPG based on Robert Jordan's fantasy book series. 

Gekkeiju Online
Graphics: Yes
Download: Yes
Free or Pay: Free
Free fantasy MUD (Multi-User Dungeon) that boasts 3D graphics. Has been running since 2003.

Lands of Hope
Graphics: No
Download: No
Free or Pay: Pay
Lands of Hope is a multiplayer browser based game, also known as a persistent browser based game (PBBG).

Multi Users in Middle Earth
Graphics: No
Download: No
Free or Pay: Free
Free text-based multiplayer RPG based J.R.R. Tolkien's Middle Earth. 

Achaea
Graphics: No
Download: No
Free or Pay: Free
Incredibly deep MUD that has been open to the public since 1997.

3Kingdoms
Graphics: No
Download: No
Free or Pay: Free
The world centers around the town of Pinnacle which sits between the 3 enormous realms: Fantasy, Science, and Chaos.

Terra Firma
Graphics: No
Download: No
Free or Pay: Free
Old school MUD (Multi User Dungeon) that tries to capture your imagination with text and excellent game-play instead of jaw-dropping graphics.

Aetolia
Graphics: No
Download: No
Free or Pay: Free
Factions of darkness struggle against each other.

Medievia
Graphics: No
Download: Yes
Free or Pay: Free
Free text-based multi-player virtual environment in which users play the role of a character in a highly-developed Medieval fantasy realm.

Kingdom of Heroes
Graphics: No
Download: Yes
Free or Pay: Pay but free during certain times.
The game world incorporates a living breathing city where new stories and plots develop each hour. The dungeons are sprawling and jam packed with monsters, tricks, traps and puzzles. The land is rich with quests that can take weeks to solve and that link together to form epic stories.

Lusternia
Graphics: No
Download: No
Free or Pay: Free
Text-based MUD set in a multi-planar universe where factions battle over reservoirs of magical energy.

Shadows of Isildur
Graphics: No
Download: No
Free or Pay: Free
A roleplaying-intensive, completely free and non-commercial Tolkien-based MUD.

The World Called Hollow
Graphics: No
Download: No
Free or Pay: Free
Established web-based RPG where graphics are replaced by intense role-playing. 

Realms of Chaos
Graphics: No
Download: No
Free or Pay: Free
Very long running text-based online RPG.