You wouldn't think of it to look at it, but the Raspberry Pi can make a pretty decent DIY smartphone. Instructables user Mach_5 shows you how to do it.
Mach_5's build uses a Raspberry Pi Model A+, a small touchscreen, camera, Wi-Fi adaptor, GSM antenna and a few other parts. It then runs a mobile operating system called TYOS to power the smartphone capabilities. Obviously, this isn't as powerful, sleek or feature-packed as an actual smartphone, but it's an interesting project nonetheless. It's also a bit more useful than previous versions of the same idea.
Build Your Own Smartphone [Instructables]