Here will follow a (very long) list of the apps I most like to use ...
Camera+ - free app. I use this as my camera rather than the actual iPhone camera
Instagram - I could not live without this app!
PSExpress - free mobile version of Photoshop!
Colour Splash - easy app for black&white with a touch of colour
TiltShiftGen - for getting that 'depth of field' look
BeFunky - the first photo app I downloaded, free, and still one of my favourites
Pixl-r-omatic - great effects
Squara - a new one, not used it much yet
Super 8 - great app for vintage looking videos!