A Windows-only 3D fantasy MMORPG, developed by Turbine Entertainment Software and published by Microsoft (who also maintain the servers as part of the MSN Gaming Zone) and released in November 1999. It took 40 months to develop, and there was then a beta period which took another 8 months.
The server technology is designed to be reusable, and features dynamic load balancing; ie, each server runs a portion of the world, and this portion changes depending on how many players are in each part of the world, in order to keep things running smoothly. This also means that the world is distributed among several servers, and hence, unlike Ultima Online and such games, there is only one world. The technology involved as a whole is very impressive, actually, and if you're interested, there is a great postmortem on Gamasutra.
The game is set on on a massive island called Dereth, which is 576 (24 * 24) square miles in size. The plot, starting with the idea that you were 'called' to the world by the wizard Asheron, improves every month with new additions, bosses and battles.
The character creation process is very flexible, with over 100 million possible different visual combinations, but only three races are involved. The innovative allegiance system allows players to form feudal-like pyramid organisations, with players volunteering to guide and help others in return for a share of their experience points, and the fellowship (aka party) system allows players to group together, with a leader, for the period of a game session.
The combat system was very innovative for the time, involving a lot more strategy and making use of physics. The spell system is modular; you make up spells by combining words, gestures and spell parts.
Player vs Player is only possible if you sign yourself over to the "dark side" by making a pact which reverses the effects of a spell designed to stop players fighting one another. Only people who have made this pact can be fought by others who have made the pact. This basically gives people a choice as to whether to participate in PvP or not, a very nice feature.
An expansion pack, Dark Majesty, was released in late 2001. It includes the new island of Marae Lassel, ravaged by many strange experiments (with new towns, quests and treasures), improved graphics and customisable player housing, ranging from small cottages to large mansions (a chest inside the houses provides secure item storage for players).
3D accelerator cards (using Direct3D drivers) are supported and are seriously recommended. Dark Majesty requires a 3D accelerator card.