I'll cop to being a magic trick geek. I'm no good at performing them, but I love the mechanisms, and used to try to create new tricks and presentations as a kid (and, OK, later in life as well). I'm surprised it's taken this long for there to be a software-based trick for the Mac, or at least, one that really was worth using. But macDazzle is offering one trick now, and another that looks even more promising later.
iPick, available now, is a classic card reading trick. A spectator picks from three distinct cards and you guess their pick. What's neat about this is that the spectator can download the app on their machine, and you can do the trick via iChat (or Adium, but the presentation isn't nearly as cool).
iMath, available "soon," looks just like the calculator widget from Apple, but it again allows you to divine the numbers provided by a spectator. To further enhance the trick, the spectator can perform calculations with the calculator, and you'll still be able to read the number, without ever touching their machine.