[UPDATE:] See how my daughter also got on the ColorSplash bandwagon, using an original first-generation iPhone: http://thesmallwave.posterous.com/colorsplash-redux

I've been playing a bit with this app on the iPhone, and think it's simple, easy, fun, and makes the iPhone pictures look better. 

While the iPhone's camera is pretty good as these things go, its pictures can be made more dramatic via this app. I tend to think of its colorization as the equivalent of using bold or italics in text. It draws the user's eyes to what you want to emphasize:

In fact, it's even useful for photos taken with other cameras that you've transferred to your iPhone, like this:

It's brilliantly designed (the brush is your finger; since it can't change size you make the picture itself larger or smaller), very well documented with help videos, and just works.
 
If you use the iPhone's camera and share photos, you should look into this app. http://www.juxtaposer.info/ColorSplash.html