If you like producing B&W images from your iPhone photo's then check out Monochrome Photo (App Store link). Priced at 59p this app allows to to create B&W images from your photo's, but also allows you to create tinted variations, which includes sepia.
This app is similar to Vint B&W (App Store link) which is also priced at 59p, but unlike Vint B&W, Monochrome Photo has more filter variations on offer, so for me is better value for money. If you were lucky enough to get Vint B&W when it was first released for FREE (as I did), then unless you want these tined options I would recommend staying with Vint B&W. If however you have neither of these apps, then on paper I would go for Monochrome Photo, which its extra features.