Just introduced these to my D&D 3e group last time we had a session. Actually, I just printed myself a deck using the transcripts for the Whimsy Card deck I found on the 'Net. These cards represent various plot complications the players can put into effect to either make their PC's life easier or harder. I'd much prefer them to make their lives harder, of course. So, I deal them one card each from the deck and told them they can change the way the story is going by playing their card and describing the actual effect. If the change is interesting, they may draw a replacement card or get some XP bonus.
They actually seemed to like the Whimsy deck, and I could almost feel them tensing as they watch out for an appropriate moment to play their card and "ruin" the plot. An amusing thing happened when they had to infiltrate a gnoll encampment. The rogue had to negotiate his way up the gate and try to unbar it from the other side. Unknown to him, the guard on the other side is Mr. Enthusiastic Gnoll. Luckily, one of them managed to play his Whimsy card which read "Sloth", a card that deals with someone's laziness! Well, that let the rogue off the hook. He chooses to investigate the loud sound of snoring though. Soon he has sneaked up to the gnoll and is preparing for a sneak attack when his player suddenly played his card: "Something's Missing". The rogue's rapier is not in its scabbard! Eventually, though, he managed to finish off the sleeping guard and let the rest of the party into the compound.
Everyone agreed to continue using the Whimsy deck in our D&D sessions.
TSR also did something called a Fate & Fortune Deck. It's included with the Mystara Player's Guide. I'd like to get a copy of those cards or if not, only the transcripts. Anyone out there?