1. First up is Percolator. It's a "photo mosaic" app that costs $2.99 and makes serious awesomeness like this shot of Saint Mark's Basilica:
Flickr and search for Percolator, you'll see a ton of other wonderful works done with the app.
2. Next up is WordFoto which can take your image and turn it into a word collage. It comes with a lot of preset formats, but there is plenty of opportunity to "fine-tune" your particular image into something perfect. You can create your own list of words and change up the fonts like I did with this Basilica image to make it reflect special details about my trip:
3. The next fun one is ToonCamera, which at $1.99 is worth it for the cool images I've come up with using the various tools and styles available. It basically takes a photo and makes it look like a sketch or a painting, depending on the "art style" you choose. It's very easy to pick options and the strength of the look, so you can really play around with your image. This is a view of Venice from the bridge to the Academia...
4. Last but not least is Color Splash that allows you to selectively colorize or un-colorize parts of your photo. Once you get used to panning and zooming around on your image, you can "brush" the color on or off the photo easily. This probably isn't the greatest example but I like how the greyscale of the buildings on the side of the lagoon contrast against the bright blue of the waterway with a few red highlights on the boats and the docks.
For only a few bucks, you could have hours of adventures right at your fingertips...making cool artwork out of your own simple photos. It's a lot more fun than solitaire, I swear!