Might be old, but what's your favorite torture you play on your players?

[Li Shenron]

No food allowed at the gaming table.

 

[nevin]

I love to change my players outlook based on what DM is doing and has done. With some DMs I've had them scared to death I was going to blow the plot line. He really started to sweat when the character started talking about killing the entire city leadership and replacing them for the common good. My player saw so many over the top bad things happen in such a short time that just killing all the potential bad guys and starting over seemed reasonable. When dozens of people are dieing everything the party does anything to try and make things better I think your average person would go nuts and in those situations my characters do unless im playing a pally

 

[John Dallman]

Sleep deprivation. Most players know what it's like, and a bunch of D&D pixies or similar with illusion powers and a cruel sense of humour can keep a party in the wilderness awake and worried all night with ease.

 

[darjr]

rolls dice behind screen
say, “Are you sure?”

 

[TarionzCousin]

Write a note on a piece of paper that says, "Read this, and hand it back to me. You cannot tell anyone what this says". Fold the paper in two so that the writing is on the inside. Hand it to a player.

One of my friends gave a note to the GM that said "Please wait a couple minutes, then say to me 'It works; you feel powerful.'" The GM did just that and most of the other players in the game wondered what exactly had happened.
N.B. They never found out.