There is a confusing range of over a dozen ESP8266 based modules available, mostly designed by AI-Thinker, all with varying numbers of IO pins, form factors and antenna types, so I'm going to stick to just the significant ones.
There is also now a range of boards designed for development which include such features as voltage regulator, USB serial port and LED indicators. In many cases these are essentially break-outs for an AI-Thinker board so you can develop and debug with the board before going into production with the standard module alone.
The prices are typical current prices from China from the usual suspects like Banggood, AliExpress, DX and Ebay. Note that Banggood now offer shipment from a local warehouse (Greenford in West London as it happens) at about 20% more but you get it in three days rather than three weeks.
- ESP-01 - under £1. The classic. Only three usable GPIO pins but in the IoT world that's enough to read a temperature sensor and drive a display.
- ESP-07 - around £1.50. Even smaller but seven IO pins, sadly/surprisingly not including the analogue input. Ships with solder pads rather than pins.
- ESP-12 - around £2. Still small but more I/O including the analogue input. A metal shield makes it allegedly FCC approved. Ships with solder pads rather than pins. Choose the later -12E variant.
- ESP-201 - around £3. More pins and both internal and external antenna connection in a more prototype-friendly format.
- Olimex MOD-WIFI-ESP8266-DEV - around £5. Little more than a breakout board, it requires FTDI-type USB-serial and is shipped with solder pads though pins can be added.
- Adafruit Huzzah ESP8266 breakout - around £7. Slightly more functional than the Olimex, this is based around an ESP-12 module it has a voltage regulator and Reset/Flash switches but still requires external USB-serial and again is shipped with solder pads though pins can be added.
- NodeMCU - around £4.50. A development module consisting of an ESP-12E on a prototype-friendly breakout board with integrated SiliconImage microUSB-Serial and voltage regulator and onboard Reset/Flash switches to assist programming. Develop on this and then embed with the ESP-12 alone. This is my favourite and I recommend it highly.
- ESP-201 Dev/SDK - Around £12. A full evaluation/development/education board with RGB and white LEDs, sounder, relay and temperature/humidity sensor, mini- and micro-usb and switches. With an ESP-201 module at the core it's ideal as a very first board or for the educational environment but little use in the longer term.