Well, it's not quite the rumored iPad 3D we heard about a while back, but this may be close enough for now. It's a free app that does 3D head-tracking, called i3D.
That's really not as amazing as it sounds. It's nothing more than a a three-dimensional image (not to be confused with stereoscopic) that displays on your iPad 2 or iPhone 4 screen. It uses the front-facing camera to follow your head around the room as you move, adjusting the flat image's perspective for your position. This creates the illusion of depth.
It's similar to the Nintendo DSiWare game Looksley's Lineup. It's also somewhat similar to HoloToy, the difference between this one an HoloToy, however, is that HoloToy uses your iDevice's accelerometer data, meaning it can be used with previous models as well. This i3D app only works with a front-facing camera.
Still a fun idea for some, and it's free. Also, we notice that these type apps seem to work a little better if you close one eye. Somehow, the effect just has a bit more 3D-ness to it because you can no longer judge the true depth of the onscreen objects.
There are also interesting applications such as the 3D Scanner app (link), which actually takes a 3D image of an object based on lighting and shadow information.
Available here: i3D
Or, you can try Holotoy as well: here.