1. Pic Stitch. This app allows you to make collages with your iphone photos. And it's free! It's very straight forward and easy to follow. The app offers effects, too - I used the same effect for all four photos so they'd look cohesive.
2. Impression. This app allows you to put a watermark on your iphone photos. There are many different fonts to choose from, and you can move the watermark around to place it anywhere on your photo. You can also make the watermark as large or as small as you'd like. This one costs $1.99.
3. 100 Cameras in 1. This app has TONS of effects and filters, and you can continually keep adding effects until you're happy with the results. Each time you create an effect, you can continue with that altered image and the app will provide 100 new filters. Once you're happy, you save the image. It gives very "artistic" effects to your photos. I paid for the $1.99 version because it got great reviews; the free version didn't have very good comments. My image is rather dramatic and needs work, but it gives you an idea of the grungy, artsy effects you can achieve.
4. BeFunkyFX. This is another app that has different effects to choose from, and I downloaded the free version which gets great reviews!. It's very simple and straightforward.