Instagram. Hipstamatic. >Good "effects" filters.
Camera + Pro. NOW THIS IS THE #1 app for smart phone camera utility....THIS IS THE ONLY APP many people will want or need. I use it. It's on the list of top 10 apps that Popular Photography & Imaging magazine oput out last year. It has a GREAT feature called Big Button, which turns any spot on the picture area into the shutter release. AND, even more important than Big Button, it has an anti-shake, motion-sensitive "shake elimination system" that truly works! Also has grid lines. THis allows you to pull out the phone, and take ONE,single shot,even at slow speeds and KNOW the shot will not be blurred due to camera motion. Camera + Pro is the single most-useful smart phone app I have ever seen. I LOVE IT.
It can also "add video capture" capability to older-gen iPhones that can not shoot video natively. Seriously.
Fast Camera. Also has a variable setting interval timer feature. Turns the iPhone into a high-speed camera, if needed, if desired. Gets the camera shooting "NOW!" as well. Shoot up to 800 frames per minute at VGA. Offers export of still frames to video as an optional feature.