ESP-8266 NodeMCU development board
The easy way to get into ESP8266 prototyping.
This is a NodeMCU v3-compatible board. v3 is a slight - though unofficial - improvement over the v2, and has some tweaks and enhancements to make it that much more useful.
It carries an ESP8266 module with 32Mb (8MB) flash and antenna on-board, as well as an AMS1117 voltage regulator, meaning you can run this board off a 9V battery.
This board uses the CP2102 USB-to-serial converter - no additional drivers required on your computer. Another nice little trick is that you can re-flash this board without needing to press the flash/reset buttons; it's a single button click from the Arduino IDE - it doesn't get better than that!
The board comes with Lua interpreter firmware pre-flashed, but other firmwares are plentiful and it's easy to set up your Arduino IDE to code it directly.