Unique items ("green items") are items with set weapon modifiers and upgrades that are randomly dropped by certain (usually boss) monsters. Since many unique items have their own special appearances as well as having max modifiers and upgrades, these items can be very useful. Unique items were introduced in the 7th of September 2005 Sorrow's Furnace game update.
All non-Shing Jea/non-Istan unique items...
Some monsters have a chance of dropping unique items when killed. Most of these monsters are bosses. Some bosses can drop multiple different unique items. Bosses often drop more than one item upon death, so it is possible to receive more than one unique item at once. Of course, you can always trade with other players to get a unique item.
In the Prophecies Campaign, unique items can be found in the following areas:
In the Factions Campaign, many unique items may be found throughout the continent of Cantha. Additionally, unique items in Factions may be traded from certain collectors (in the Divine Path) using an Amulet of the Mists.
The same is true in the Nightfall Campaign, as many unique items can be found across Elona. Also, like Factions, unique, end-game items may be traded from certain collectors (in the Throne of Secrets) after the final mission using a Book of Secrets.
Also in the Eye of the North expansion, unique items can be found across Tyria and dungeons. End-game items may be traded from certain collectors (in the Epilogue) after the final mission using a Droknar's Key.