If you are looking to add some creative fun to your iPhone photography, then you should check out Dubble. It's a collaborative social network for photographers that allows you to create weird and wild mashups by combining your photos with those from strangers.
Before you get started, you should know that Dubble is a gated community -- you can't check out the photo pairing without signing up for the service by providing and email address and creating a login. Once you have joined, you can start "dubbling" your photos with others.
Dubble allows you to upload a photo from your camera roll or take one using the app. The app then combines your photo automatically with another user's photo to create a multiple exposure image. The results are sometimes pleasant, sometimes unusual. Dubble encourages you to mix your photos with random strangers, but the latest version of the app now provides an option to mix your photos with just your favorite Dubble users (premium feature) or only yourself.
Dubble is a social network at its core, allowing you to create your own profile and establish your own identity in the community. As a Dubble member, you can browse through the growing library of dubbles, favorite other Dubble users to follow their activity, and comment or like existing dubbles.
Dubble is available for free and is compatible with the iPhone running iOS 7.0 or later. The app requires an account and offers a 99-cent in-app purchase to enable dubbling with your those on your favorites list.