There’s an article on LifeHacker with a list of what the author thinks are the best photography apps for the iPhone.
There’s a link in the article to another article with photography apps for Android users.
I have Hipstamatic that is mentioned in the article. I also have MillColor which is not mentioned but which I like a lot.
It lets you tone, tint, cross-process, and generally do a lot of nice things with your photos. This is a photo I just took with my iPhone. It is, as you can see, a photo of an old pair of glasses. I then processed it with MillColor using the ‘chocolate’ filter.