There's many 'toy camera' apps for iPhone and Hipstamatic is probably the most popular. This Retro Camera Plus is a very strong competitor in my opinion and it's free! In some ways it's even better. I like the fact that you can't choose film and lens separately with Retro Camera Plus. There's too much combinations of film and lenses in Hipstamatic for my simple mind and it's also slower in processing than this.
I don't like 'Pinhole camera' and 'Little orange box' in this app because the frames are a bit over the top for my taste. 'The Bärbl', 'Xolaroid' and 'FudgeCam' are great. 'FudgeCam' comes probably closest to a real toy camera like Holga as I have seen in any app.
The key to any kind of toy camera type photography is to keep composition as simple as possible. That is even more critical with cell phone cameras because depth of field is infinite due to the small censor.
Here's few pictures from winterly Helsinki with Retro Camera Plus.
Above pictures are all taken with the 'The Bärbl' setting.
This one was taken with 'FudgeCam'.
This installation was in a court yard of a mental institute. I have no idea what is going on here. Maybe they run out of place to put their laundry to dry.
Color photographs above ware taken with 'Xolaroid'.'
'Little orange box' effect here.
'Pinhole camera' sample.