Booster! from misskiwi, the developer behind successful apps such as Squara, ClassicINSTA and ClassicTOY, is a new app that lets you create your own live camera filters.
The app, whilst simple to use, is very powerful. Optimised* for the iPhone 4S (although compatible with the 4), Booster! lets you select 1 of 7 pre-defined filter effects from B&W or Sepia to Cross Process, which you can then adjust the Saturation, Contrast and/or Brightness, add some Vignette, or increase/decrease the filters Colour Temperature, before using to take photos with the apps camera, and if these customisable filter adjustments weren't enough, the app has one more powerful trick up its sleeve. From the filter options, choose "Normal" (which is no filter), and then use the "Curve" option to create your own unlimited filter effects, which again you can adjust the Saturation, Contrast and/or Brightness, as well as the Vignette and Colour Temperature.
*As mentioned above, this camera only app is very powerful, and saves at full-res when used with an iPhone 4S, sadly will only save at 1530x2048 pixels on an iPhone 4, which is about 2 megapixels less than it is capable of. All that said, if you're looking for a powerful and customisable iPhone only camera replacement app, then at its current sale price of £0.69/$0.99/€0.79, Booster! may be the app you've been waiting for.
App Store Link: Booster! Sale Price: £0.69/$0.99/€0.79