Expansions
Since it's initial release, along with 3 "Anniversary Collections", UO has known over 7 commercially released expansions with another planned within the next year.
Stygian Abyss
Scheduled to be released sometime after Kingdom Reborn, Stygian Abyss is to feature gargoyles as well as the Stygian Abyss itself as a massive dungeon with at least eight levels. Not much else it known about this expansion so far, but hints have slowly been trickling out the past few months.
Kingdom Reborn
Scheduled for release in the second half of 2007, Kingdom Reborn is going to introduce a modern client with 3D graphics. Not much else is known about it at this time, but more information should be coming before the end of 2006.
Mondain's Legacy
Released on August 30 2005, Mondain's Legacy is the most recent expansion. The developers attempted to get back to UO's roots by focusing on current content. NPC Quests were added throughout the land. New dungeons were integrated into the landscape and the village of Heartwood. The only new skill was Spellweaving.
Samurai Empire
Released on November 2 2004, Samurai Empire was is considered a modest expansion compared with its predecessor. It added the Tokuno Islands land mass. The skills of Bushido and Ninjitsu were added. Another feature of note is the addition of roofs to House Customization.
Age of Shadows
Release on February 28 2003, Age of Shadows is considered the most impactful expansion in UO history. It completely rewrote and changed many systems, including the introduction of Item Properties, Item Insurance, House Customization, and more. The skills of Chivalry and Necromancy were also added. It is also the most critizied expansion due to many of its features.
Lord Blackthorn's Revenge
Released on February 24 2002, Lord Blackthorn's Revenge was the forth expansion and the first to come with no land/dungeon expansion. Many of its creatures were created by Todd McFarlane and used in Scenarios. The Ilshenar facet was also opened up to all 2D Client users.
Third Dawn
Released on March 7 2001, Third Dawn changed the way player saw UO by adding the 3D Client. It also introduced the land of Ilshenar which was only available to 3D users until the next expansion.
Renaissance
Released on April 3 2000, Renaissance was UO's second expansion. It completely changed the way the game functioned with the introduction of Trammel. Player interaction and dependence were decreased significantly. Other major changes included the introduction of Factions and Combat Moves.
The Second Age
Released on October 1 1998, The Second Age (T2A) was UO's first expansion. It was also the last project led by Richard Garriott, the original creator of UO and the Ultima series. T2A mostly added on to the existing game without changing any core systems or concepts. It introduced The Lost Lands to the world along with many dungeons.