Photo Search
I like to play a game with my young daughter when I’ve got my laptop. She names anything, usually an animal, and I bring up a picture of it by searching the net. She says “buffalo” and I can instantly show her tons of buffalo pictures. She loves it, and its educational. This is also useful for adults when you want to explain or demonstrate something. You can find pictures of almost anything on the net.
With the iPhone, you don’t even need your computer. You can always use Google image search to do this. I also found a Flickr photo browser that has an iPhone interface.