Citadels is an easy but fun card game where the players compete to construct the most impressive city by amassing wealth to spend building various districts (docks, university, cathedrals, etc). The game ends when a player plays an eighth district then everyone's city is scored (and certain bonuses added) to determine the winner. Simple.
Or 
not so simple. There are 8 role cards, each player will get one of these each turn which will enable certain actions. For instance, the Magician role can swap hands with another player, the Architect can build more in a turn, the King gets first choice of roles for next turn, etc. Because there are more roles than players and the roles are chosen secretly in turn, the way to win lies in choosing the correct role at the correct time. Some of the more expensive districts also confer additional bonuses apart for score such as more money or protection from certain attacks so thinking several turns ahead is required.
Citadels can be quite a sneaky game - many of the roles allow you to ruin your opponents plans by stealing cards or money, or even destroying their hard won districts from under them. But it is hard to get an unassailable lead and the way the roles work means that no player can really feel ganged-up on. It is also one of the few games I have played that actually works better as a 4 or 5 player game (haven't tried 6 or more) without leaving some players in an unwinnable position.
The game itself is attractive and the cards are well designed. The one flaw is that the role cards get constantly handled and can get bent or scuffed up, which is a problem since they are supposed to remain identical to maintain the secrecy required. The basic game is flexible, the official website has a whole bunch of alternate rules to turn it into a children's party game or a drinking game (although hopefully not at the same time). With 4 or 5 players the game takes about an hour to play.
Highly recommended.