For first time customers to the OEMOS Flagship Store on AliExpress, they are currently offering a number of discounts and one of them is this development board based on the ESP8266 chip.
It is currently selling for around $5 (AUD) and they will give you a discount to make it cost only $0.59. But for some reason, AliExpress insisted on charging me $0.52 GST so the total price I paid was $1.11 (free postage from China). I ordered it last night and it was shipping from China this morning.
This board is in the Arduino Uno R3 form factor with the female headers and is compatible with Arduino Shields. However, the chip is different from the official Arduino; it is a lot more powerful and you get WiFi. I have never used it before but from what I understand, you just have to load the appropriate ESP8266 libraries to get your Arduino programs to run.
I believe you need to be completely new to Aliexpress to get the welcome voucher. Existing Aliexpress accounts with history of purchases from other stores aren't eligible.