If you're looking for a plethora of filters to apply to photos, Camera Plus
is the best free option. The lens built into the app has a grid, making it easier for you to shoot straight images and set up a well-composed shot, and a timer lets you schedule the snap of a photo in five, 15 or 30 seconds. The app then lets you crop images, adjust lighting, apply one of more than 25 filters, add borders, and more. You can email photos or share via social networks from within the app. Fair warning: Camera Plus' many editing options make it easy for you to spend way too much time tinkering and adjusting.
Available on iPhone, iPad, iPod Touch