My friend used an iPhone for his DIY project, but the idea should work just about any smartphone that has a built-in camera. He calls it the Phone-O-Scope, a smartly cobbled together out of things you may already have laying around the house (assuming you also have an SLR lens from a full-size camera).
Using little more than a cardboard tube, some sticky putty, duct tape (of course!), and a couple of PVC pipe couplers, he made his Phone-O-Scope for a mere 10 bucks. Check out his blog post for step-by-step instructions on how he did it.