Since its initial release, along with 3 "Anniversary Collections", UO has known over 7 commercially released expansions.
Released in September 2015, it featured new lands The Valley of Eodon and added the Skill Mastery System and completed the Virtue System. as well as adding new monsters, armor and simplified the currency system.
Released in October 2010, Sosaria was beset with pirates, cannon, new ships and even a floating village. This was the first "booster", which costs roughly half the price and features new, but less, content than a "full" expansion. Boosters are supposed to be released roughly every 6 months. The Fishing skill was greatly expanded with new rewards and opportunities.
Ultima Online: Stygian Abyss was released in September 2009 and features a new playable Gargoyle Race as well as the Stygian Abyss itself, a massive dungeon with several levels. Popular highlights include the Imbuing crafting skill, the Mysticism school of magic spells, the new landmass of Ter Mur and new champion spawns, artifacts, rare stealable and dungeon puzzles.
Released in 2007, Kingdom Reborn was a graphic and user interface upgrade, an attempt to offer a modern MMORPG client that could also run on lower-end computers. It was released through a free download, though for those who did not want to bother with the download a disk is available for purchase.
Its level of success and failure are unknown by players, and the wars over its merits or demerits are waged on message boards like Stratics with a fevered intensity.
KR was eventually replaced by the Enhanced Client that came out with the Stygian Abyss expansion. Most things in KR are now in the "Enhanced Client" or "EC".
Released on August 30, 2005, Mondain's Legacy was 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 elf race and their village of Heartwood appeared. The only new skill was Spellweaving.
Released on November 2, 2004, Samurai Empire was 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.
Release on February 11, 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 criticized expansion due to many of its features. The new land of Malas was added with this expansion.
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.
Released on March 27, 2001, Third Dawn changed the way player saw UO by adding the 3D client, though the view was still fixed angle isometric. It also introduced a new land mass (Ilshenar); several new dungeons; numerous new creatures; several new armors (e.g., samurai, ninja); as well as four new weapons.
Released on May 4, 2000, Renaissance (UOR) 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.
Released on October 24, 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.