Combo: Black Market and Tactician
From DominionStrategy Wiki
Black Market and Tactician is a notorious Dominion combo. Black Market allows a player to play treasures before his Buy phase. This allows a player to play a Black Market followed by a Tactician every turn, getting to use his treasures while getting the 5 card bonus from Tactician. This combo can be very effective, even without buying anything from the Black Market deck.