Raspberry Pi Zero
Here really is the most unique and most pared down Raspberry Pi up to now. It is sort of like the small cousin into the Pi - using only a micro SD card slot, a miniature HDMI interface, two micro USB interfaces (one for power, one for USB), and 512MB of RAM. It's a single-core 1 GHz processor chip, like the Pi A+ and B+.
The best thing about all this is the Pi Zero W retains the exact same form, connectors, and mounting holes since the Pi Zero v1.3. 99% of accessories and cases will still be entirely compatible with the Pi Zero W and v1.3 - however in the event that you've got a situation with a metallic top there could be several WiFi chip problems.