The Raspberry Pi Zero is a serious, not-even-kidding $5 computer you can buy right now and put into all sort of projects, but to get it connected to the internet or wireless peripherals you'll end up spending at least $20 or $30 on cables and dongles. Really, if Wi-Fi and Bluetooth are important to your project, you're probably better off getting a full-sized Pi... until today. Now the Pi Zero W has arrived, and it changes the whole game.
The Pi Zero W adds the same 802.11n and Bluetooth 4.0 wireless chip the Pi 3 Model B has, but in that deliciously small Pi Zero form factor, and for only $5 more. I know it sounds like I'm trying to sell you one of these, but really I'm just regretting my recent purchase of a Pi Zero starter kit, USB hub, Wi-Fi dongle, and Bluetooth dongle. At least my HDMI adapter is still useful!
Otherwise the board is the same as the Pi Zero: a single-core 1GHz CPU, 512MB of RAM, and plenty of I/O for hacking.
You can get a Pi Zero W today if you can find one in stock anywhere, but they're disappearing fast. The good news is the Raspberry Pi Foundation usually, eventually, catches up with demand.