Tribes RPG is a medieval themed free online RPG (similar to free-to-play MMORPGs, although not quite as many players).
Tribes RPG is a total conversion/modification for Starsiege: Tribes which converts the First Person Shooter into a Role Playing Game using swords, magic, and bows instead of guns. Generally on RPG servers you select one of several different classes, which usually determine how quickly different skills advance, and certain traits (rangers can sleep outdoors). You can then explore a large world, containing various dungeons and towns, populated with various types of monsters and NPCs, respectively.
Combat is primarily PvE, meaning you fight monsters instead of the other players, although there are PvP areas.
Tribes RPG was originally created by someone named Mathieu Bouchard (who identified online at the time as "JeremyIrons"). He left the scene around 2002, and around the same time I came along and started hosting a server.
Most RPG mod servers are unique, as Tribes and the RPG mod are naturally very modifiable. Join an established world with other players, or make your own!