Above is one of the earliest photos in history and was taken in 1838. It took more than 10 minutes to properly expose the image because the sensitivity of the plate (not film! not yet invented) was so low. This is why the street is empty except for the only two people (who did not have any big movement during the exposure) in lower left of the photo. Today, even with ASA25 film, you would probably need a 10 stops ND filter to make an exposure that long in daylight!
Anyway, this post is not about history of photography but about how to have fun with digital retro-style photography. A while ago, I found an iPhone app. named Retro Camera (available for both iOS and Android) which can mimic the results of several classic cameras including a pinhole one. It is fun to use because you can skip all the post-processing and get the result immediately although you have to stick with the styles that the app. provides. On the other side, you can use one of the many on-the-phone photo editing software to give your camera phone shots a taste of retro.
Photos below were taken with the 2 megapixel camera of the iPhone 3G through the Retro Camera app.
And the one below was created with an on-camera photo editing app. called Best Camera.