Apps to look for:

Photo Booth Classic Plus

Art Camera processes your shot into the painting style of various famous artists.

PhotoFunia offers a multitude of different scenarios into which you can put your own photos.

There are plenty of Photo Texting Apps, too

Nikon Learn & Explore is a photography tutorial right there on your iPhone.
makes a four-shot strip of snaps in the old "photo booth" style.

Social Apps

Access your favorite photo storage sites with Flickr, Photobucket,
Pixelpipe and PicasaFoto

There are also a number of apps which facilitate the sharing of your images through social networking sites like Facebook and Twitter.