Action - Attack
Effect: +1 card, +1 action. Reveal the top card of each player's deck, and choose whether it stays or is discarded.
Spy is a card of mild advantage. Unlike other attacks guaranteed to curse your opponents, or gain you some spending power, spy's only benefit is the ability to look at the top card of each deck and decide whether it stays or goes. In many cases, this will be useless. However, the +1 card, +1 action means that playing a spy has no cost, so with even this small, unreliable advantage, the spy can still sometimes be worth putting in your deck.
Spy isn't a card you should generally bother with in the early game, absent special circumstances. The attack often isn't very harmful to your opponents, and the benefit to you is minimal at best, which means you'd be better off opening with another card that boosts your deck instead -- even Silver, if no suitable $4 card is available. By the mid-game Spy starts to become mildly useful and stays that way through the endgame.
In a 2-player game, Spy generally isn't worth buying unless you're going to get a lot of them and create a "Spy network" deck where you play many spies every turn. (Even then, the advantage of such rarely outweighs the annoyance of playing spy after spy every turn.) In a 4-player game, however, your odds of denying someone their best card improve somewhat, making Spy a reasonable, albeit not powerful, option.
Spy is not a strong card in a vacuum. The advantage of playing Spy, unsupported by a combo, is often fairly minimal. It never hurts you to have more spies in your deck, so it's something you can buy without worrying about it being a dead card. But before you buy a spy, ask yourself: Would I be better off just getting a silver? The answer to that question is often yes. All that being said, if you can pull off a Spy network and prevent your opponent from drawing his single Gold card, they can be effective.
Spy networks (Spy after Spy after Spy) aren't bad. Each spy gets a chance to discard junk from the top of your deck before the next spy draws you a card, and by the end of a 3+ Spy turn, you usually have decent control over what might top an opponent's deck.
Thief and Spy are best friends. Spy to see if any silver or gold might be on top of opposing decks, and if so, play your Thief for profit. If not, you have two choices: Discard the top cards of opposing decks to increase the chances of your Thief working, or leave bad cards atop opposing decks and play a different terminal action instead of the Thief. Of course, if you have many Spies, you can dig for silver or gold for your thieves.
Council Room is a fine play to follow your Spy. Ideally, you've left your opponents a lousy sixth card to draw, and left yourself something good.
Moats in the hands of your opponents can stop your attacks cold. But Spy gives you information -- if your opponent reveals a Moat when you play your Spy, you may not want to waste your terminal action on an attack like Witch that can be blocked -- play the Council Room instead!
Moat in hand, as always, stops the spying.
Chancellor will dump your deck into the discard pile, if you know your top card is junk.
Buy multiples of the important cards in your deck -- Spy hurts most when it can skip your single copy of a powerful card.