The Pi Zero was released back on late 2015 as a cut down Raspberry Pi model A, for a price of USD5. It lacks Ethernet just like the Model A but goes further and removes the usb hub, composite video jack, and even comes without GPIO connectors soldered. At less than half the size of the Model A, it's a great proof of concept of what you can do for USD5, but at its time of launch I didn't see much use for it ... without ethernet it's not going to be an Internet connected device, and even though you can connect it to a TV there's not much to see offline. You can add all the missing features via USB (hub, ethernet, wifi, 3G, etc) but then you might as well buy a full sized Pi.
They recently released a minor update (The Pi Zero rev 1.3) that adds a camera port (just like on the larger Pis) but again without Net connectivity the most you can do with whatever images you capture is store them locally or display them on a screen, so again, it's no biggie.
However one feature I did find interesting was that the Pi Zero's single USB port not only acts as a USB On-The-Go port, but has OS support to actually act as any USB device. This means that it can "pretend" to be a USB device when connected to a PC, and can emulate a USB-Serial device or a USB Network device (both of which let you communicate from a PC to the Pi with just a USB cable) or even a USB Human Interface Device (HID) such as a keyboard, mouse or joystick. There's probably enough OS support that you can hack this do quite a lot of interesting things.
One of my future project ideas is to build a Throttle Quadrant for my X-Plane Flight Simulator PC. I'm currently using a simple Saitek device but building one would be an interesting learning experience, especially if you go for a motorized one. While I can interface almost everything else to X-Plane via a network connection, X-Plane only allows axis-based controls to come from USB devices. With the addition of a ADC chip I could get the Pi to read the throttle lever movements and by configuring it to appear as a USB joystick device, X-Plane would be quite happy to work with it.
(A more traditional way to achieve this is with something like a Teensy which is an Arduino-like device that can also pretend to be a USB device. It's actually much simpler to code and has the ADC built-in, but then I wouldn't get to learn much about using Linux as a USB device if I went that route).
Unfortunately ... the Pi Zero is a victim of it's own goals. At USD5 the profit margin was extremely low, so much so that it had lesser priority in the assembly lines. Not only that but the major Pi major global distributors (Element14 and RS) decided not to carry it. It's available from a few smaller distributors based in the UK and US at the official price, but with very low stock availability. And to make a bad situation worse, some don't offer international shipping and for those that do, I would end up paying something like 3-8 times the price of the Pi Zero in shipping costs.
Therefore, I mostly gave up on getting one. So much so that I ordered a Teensy a few weeks ago to play around with.
But then a few days ago I came across a local (within Malaysia) seller at http://www.raspberryware.com/ that had Pi Zeros for sale. There were priced at RM79 -- which with the currently slump the Malaysian Ringgit is in comes up to USD18. That's 3 times the original price but given that this seller has to source and import them I figured it was still reasonable. They sell Pi 3's at RM179 which is RM1 cheaper than what Element14 sells them, so I think their prices are pretty decent. They carry the full range of Pi products as well as a few related accessories.
So I went ahead and ordered one. Payment was by Paypal Only (which means you need a credit card -- it would be great if they accept direct debits as well, as quite a number of Malaysians don't have credit cards). It arrived the next day:
Pi Zero from rasberryware.com
I was quite surprised to find that it came with a microHDMI to HDMI adapter and also a USB On-The-Go cable! You need this to connect to the Pi Zero (at least initially) and although I probably have them somewhere around the house, including it is a big convenience and also makes the USD18 price much more justified.
The unit I received was the pre rev 1.3 model so has no camera connector, but as I said earlier it's no biggie.
It's much much smaller than the full-sized Pis, but if size is what you're after then the teensy is even tinier:
So now I have a choice of two gadgets to play USB device simulation with. Now all I need is more time to indulge into all this with ... at my current rate it looks like it will be 2018 before I start on that Throttle Quadrant. 8-)