Anno 1404: Venice was released on February 26, 2010. The expansion includes 15 new scenarios, an espionage system, volcanic islands, an added third non-player power known as the Venetian dignitary Giacomo Garibaldi. Garibaldi enables the player to sabotage other player's cities and/or buy them out. Originally, there were also three new computer players planned but these were scrapped for unknown reasons. The venice expansion also allows the player to play the original 6 scenarios of the base game but with all the added features and improvements of the Venice addon.
Many new buildings were added:
A multi-part personal mansion
New ornamental buildings such as Saint Mark's pillar
Two new quest type were added: ship race and privateering.
Furthermore, the player now has the option of requesting a quest in order to gain more honour points and/or gold coins/goods.
Source: Wikipedia, "Anno 1404", available under the CC-BY-SA License.