Here is a list (with links) of some of the most widely used photographic apps for iPhone. I have used these extensively and found them to work really well, either as standalone apps, or in combination with others. These are not in any particular order of importance or significance.

1. Hipstamatic
2. Plastic Bullet
3. Camera +
4. Swanko Lab
5. Cross Process
6. DXP
7. Diptic
8. Retro Camera
9. Lo-Mob
10. Cool FX
11. Photo FX
12. Dynamic Light
13. Blur FX
14. Slow Shutter
15. Noir
16. PhotoForge2
17. Instagram