The tricks just keep on comin' for the iPhone. Magicard [App Store link] is a good head scratcher that is always ready to perform. Have your assistant cut the cards a few times and select a card. (The method to do this is provided with a built-in tutorial video.) You keep the selected card face down, and place it on top of the iPhone screen. When the card is removed, a mirror image of the card is on the screen. The card can be any number or suit.
The app normally starts up with a screen that includes a clickable link to a video tutorial that shows how the trick works. It wouldn't be good for people to see that, so in your iPhone settings menu you can tell the Magicard not to display any of that information when you are about to perform.
Magicard also lets you select a plain black background for the trick, or you can use something from your photo roll. You can even snap a picture with the iPhone camera and use that. Unfortunately, this menu comes up every time you try the trick, and it is an annoyance. You should be able to set the background from the settings menu and forget it until you want to change it.
The trick does not require a lot of skill, but you will need to do one common thing magicians know how to do. If you don't know how, the video tutorial will walk you through the simple steps. I don't think David Blaine will lose any sleep over you performing this trick around town, but it is worth the US $0.99 asking price to see some surprised faces on your friends when you pull this little effect out.
Note: This trick is iPhone only. It will not work on the iPod touch. You can see some videos of the trick in action at the developer web site, along with some variations on the basic trick.