And it costs $5.
It uses the Broadcom BCM2835 processor (40% faster than the original Pi) and has 512MB of RAM.
There's a microsSD slot for the OS, a mini-HDMI socket for full HD @60 fps and Micro-USB sockets for data and power. It's about half the size of the original Raspberry Pi but has the same 40-pin GPIO header (to connect it to real-world I/O) and it includes space for composite video header as well. Raspbian Linux has been ported to it, so it should work just like the other Pi's.
It's available from Adafruit and MicroCenter here in the US, but they expect demand to keep them out of stock for a while. They say they've produced tens of thousands of the little computers, but at $5, demand will likely eclipse even the very popular Raspberry Pi 1 when it was announced.)
The A+, Pi 2 and the B+ models are still available (at $20, $35 and $25 respectively.)
With the incredibly low cost of these devices, they're likely to show up in everything from light switches to drones.
For more details: https://www.raspberrypi.org/blog/raspberry-pi-zero/