Mine

Description: Trash a Treasure card from your hand.  Gain a Treasure card costing up to 3 coins more; put it into your hand.
Type: Action
Cost: 5
Set: Dominion

This a great card by itself as it allows you to upgrade your copper -> silver -> gold.  And don't forget that it goes into your hand RIGHT AWAY.  When this card is in play, there is no longer a need to trash those coppers, but rather you can upgrade them to silver.  Get a copy of the mine card early on in the game, and you can be sure to quickly turn your coppers into gold soon.  There probably isn't much of a need to get more than 1 or 2 of these cards as it becomes useless near the end of game as you will have already upgraded most of your cards.