The urge to add vintage style effect to photographs seems to be uncontrollable for most iPhone and social network users. As a result there are loads of these special effect apps floating around in the App Store. RetoGram is one of the newest and its main claim to fame is that, well, it's free! It does limit photos to 700-pixel resolution, but that's probably enough for shots that you're going to post to Facebook or Twitter.
There are 12 free filters included with the app and you can apply two filters at a time to any picture for more complicated effects. Additional effects can be bought if you want, but the company will also release extra free filters on a weekly basis.
For a free app, RetroGram offers a good range of filter effects and also has a cute and stylish interface.