One of the coolest things we've seen lately is this projector for the iPhone 4/4S.
Made by Brookstone, the projector is incredibly easy to use! With an incredibly bright 15-lumen projector lamp, it projects up to a 50" image. And with the built-in .5W speaker, you can hear perfectly! There is also a focus adjustment to ensure optimum viewing experience.
And bonus? The built-in 2100mAh battery also works as a back-up batter for your iPhone (when fully charged).
Obviously, the picture quality will vary based on where you're using it. To get the best quality, keep the room as dark as possible. Of course, if you're using this to entertain your kids that won't always be possible. But for business presentations, definitely try to keep it dark.
The projector is a little pricey ($229.99) but totally worth it!