Set: Intrigue
Each other player reveals cards from the top of his deck until revealing one costing 3$ or more. He trashes that card and may gain a card costing at most 2$ less than it. He discards the other revealed cards.
This card is most definitely unpleasant to deal with, and I understand completely why people really don't like playing with it. It's absolutely maddening to get hit by this and lose your important cards, especially your Provinces. But what's really funny about it is that this card isn't even all that good.
Sabotaging someone's deck can clamp down it pretty well - it removes their good 5, 6 cost cards and replaces them with crappy 3, 4 cost ones. But the problem is that if you're playing Saboteur, you're hurting your own deck worse. Saboteur is a dead card - unlike most attacks, it doesn't even give a token +2$ or +2 cards benefit. It exists solely to kill the other person's deck, and it's questionable whether it even hurts their deck more than a single Curse would.
Probably the biggest reason everyone hates Saboteur, and the reason I find it annoying as well, is its wild variance. If it does hit those Provinces, then it's pretty nasty. If it just its inconsequential Silvers or whatnot you're completely wasting your time. Although even getting a Province hit isn't that bad - just take a Gold early or a Duchy late and accept the 3 VP swing, the player using Saboteur has probably set themselves too far back anyway.
Saboteur is probably best in a good engine where you aren't worried about deck clutter. Not only will Saboteur's presence hurt your deck less, but Saboteur is also a card that needs to be played frequently. If you're only playing Saboteur once every four turns, you're not doing well enough. If you're playing Saboteur every turn, now you're doing some significant damage. If you can manage to play Saboteur more than once every turn, now you're really putting the opponent's deck in a vicegrip. Engines like Scrying Pool or Hunting Party, anything that can increase handsize and draw most of your deck, are the most friendly environments to play Saboteur in, since you'll play it more often and draw it at an inconvenient time less often. King's Court is especially crazy with Saboteur though - getting to play it three times, even if it gives you no direct benefit, can absolutely trash the opponent's deck, and that may very well be worth it.
Most of the time people just don't really know what they're doing with Saboteur and buy it because it seems good, not realizing that every card in Dominion that doesn't give you an immediate effect is hurting you in some way if you're not careful. Saboteur is probably the biggest example of this in the game. It's best left alone in most games, if not for its ineffectiveness, then for the fact that everyone will hate you. But if you do get hit hard by it, don't worry - you'll probably still win.