These are all great, guys. Keep 'em coming. I suppose I should share an old favourite of mine. It’s done the rounds a fair few times so you’ve probably heard it, but it’s very effective on those who don’t know how it’s done. Its a little difficult to explain so bear with me.
Step 1: Find someone a bit drunk. Sober people tend to see through this trick pretty quickly so you want someone who isn’t thinking too clearly.
Step 2: Get three shot glasses and line them up horizontally. Turn the outer two face down and the middle one face up.
Step 3: Say to your dupe: “I bet you that you can’t turn these three shot glasses so they’re all facing up in three moves.” When your mark expresses incredulity say to him “No, seriously, it’s harder than it sounds. Especially since there’s a catch I forgot to mention. You need to turn exactly two glasses each move. Anything else is cheating and doesn’t count. Look, I’ll show you how it’s done”.
Step 4: At this stage of the game, the outer two glasses are facing down and the middle one is facing up. From this starting position it’s trivially easy to turn all glasses facing up in three moves, moving two glasses at a time. In fact, you can do it in one move but the trick works better if you contrive three moves. So turn all the glasses facing up in three moves.
Step 5: Turn the middle glass face down, so the outer two glasses are facing up and the middle one is facing down. Offer your sucker an opportunity to repeat your feat. Take a seat, 'cos you’ll be there for a while 
Why is this? Well, to recap, when you do the trick, the outer glasses are face down and the inner glass is face up. From this position, it is trivially easy to turn them all face up in 3 moves moving two glasses at a time. When they are offered an opportunity to perform the trick, the inner glass is face down and the outer glasses are face up. From this position it’s impossible to get them all face up.
Unbelievably, people generally don’t notice that you invert the glasses before you let them try. They only notice that the middle one is facing in a different direction to the outer one’s. In my experience, you need to show them about 3 or 4 times before they get it, and that’s if they’re sober. If they’re drunk you can have literally hours of fun with this. Once they’re suitably frustrated they’ll gladly buy you a drink in return for the solution 
This one’s also a great icebreaker with girls, by the way.