A number of people have asked me whether it possible to build a smartphone out of easily available parts. Absolutely. It won't be as sleek or as powerful as one you could pick up, and you will have to assemble it yourself, but it's one of those projects that's very doable.
Here's one example built around a Raspberry Pi board, and Adafruit Fona GSM board, a 5-megapixel camera, 802.11n Wi-Fi, and a 3.5-inch display. You'll have to shop around for some of these parts, but the last time I looked they were all still available.
Now, this might not be a device that you want to carry around with you as a daily smartphone, but it's something that you could integrate into a bigger project, such as an IoT hub or maybe a rugged handset for use in an off-road vehicle or boat.
Not only is this a great electronics project, but it also allows you to get your hands dirty with some coding. And at the end of the project you end up with something very unique and totally usable.