Everybody's heard of Ultima Online no doubt - the first big massive multiplayer online rpg (or MMORPG). However Ultima is used as little more than a brand name these days, the original series of games (which are for the most part excellent) are often eclipsed by the popularity of the online cash cow.
Ultima began in the days when people wrote computer games in their bedrooms and sold them via mailorder on those things they call cassettes. Richard Garriott, an Austin-based roleplayer and computer geek with the handle "Lord British" created a Dungeons and Dragons-type game for his home computer and it was called... Akalabeth. This wasn't more than a tiled adventure game where you played a stick figure, moved around the map, killed monsters and explored dungeons but was pretty sophisticated in the 1970s - particuarly the 3D dungeon bashing.
Ultima IV introduced the idea of virtues - a code of ethics that you were encouraged to follow if you wanted to win the game. This was partly in response to criticisms that the original 3 games encouraged you to be a homicidal maniac in order to complete the games, but also because the last 3 games were all about killing some big nasty.
You, a traveller in the newly formed lands of Britannia, summoned from the world of Earth by Lord British, had to excel in all 8 virtues, complete a number of quests to become the Avatar, the paragon of virtue and find the Codex of Ultimate Wisdom, a book of er... wisdom which would allow you to essentially become a spiritual leader to the people of Britannia.
Ultima IV was the first real "plot" the series had beyond killing stuff, and you had to interact with townsfolk rather than rob them blind to solve the quests. It also finally created a Narniaesque connection to Britannia - you were meant to be yourself transported to the magic realm of Britannia, which I always found rather neat. You also had to gain companions native to Britannia - there was one for each virtue. The interaction was very primitive but a lot of important characters were introduced. In particular Iolo the Bard, a handy shot with the crossbow, Dupre - who would eventually be detailed as a paladin with a love for ale and women and Shamino - a ranger and spiritualist would become the "core three" companions, whilst the other five would feature in every other Britannia based Ultima, though they wouldn't always join you.
Ultima V was set a few months later after you completed your quest and returned to Earth. Time on Earth runs differently to Britannia so a few months on Earth amounted to several years on Britannia.
Ultima V explored the themes of religious fundamentalism and legislating morality. Lord British had gone missing, a new ruler, Lord Blackthorn assumed power and began issuing laws that made following the virtues compulsory (for example the Law of Honesty, if you lie you lose your tongue). 3 dark creatures that opposed the virtues, the Shadowlords, were roaming Britannia, you and your companions were declared outlaws. It was a bad time to be the Avatar, dude, and your mission is to return to Britannia, destroy the Shadowlords and find Lord British.
The original game was another tile based game with better graphics, more sophisticated conversations and a schedule system (e.g. people go off for lunch at noon, sleep all night etc.) but it was more recently remade using the Dungeon Siege engine, and is available here. Other than infinitely better dialog, full colour portraits for every character and a 3D engine that is infinitely less tiresome than the original tile game this game also now boasts a feature that those 60 Degrees North might appreciate - the ability to play as a complete git and rather than rescuing Lord British you can make him disappear permanently, becoming a thrall to the Shadowlords and Blackthorn and ruling Britannia with an iron fist. Not a good ending, trust me.
Ultima VI was the first "real" Ultima for me - still tile based, but sufficiently polished that it is still highly playable. It involved an alien race, known as the Gargoyles, who were invading Britannia and nearly sacrificed you on an altar with a sharp pointy dagger... you get the point? However it turns out the gargoyles aren't evil, just justifiably pissed off, and you have broker peace between them and the similarly understandably peeved off Britannians.
In Ultima VI conversations were very detailed with the denizens of Britannia, there are oodles of sidequests and a massive world to explore.
Ultima VII was the best of the series. It began the last trilogy in a trilogy of trilogies.