Market

Description: +1 card, +1 action, +1 buy, +1 coin
Type: Action
Cost: 5
Set: Dominion

A very powerful card even by itself.  It replaces itself and gives you an additional buy and coin.  It is great for those long combos which give you lots of coins, but only one buy.  You really can't go wrong with buying this card in any situation in my opinion.