The ESP8266 is one of the game changing building blocks of the Internet of Things. It can serve many purposes with its Wifi functionality and has enough I/O facilities to replace an Arduino or Raspberry. Strictly 3.3V, and a bit powerhungry for battery use.
ESP-01 the ultra small board, not much I/O The first and most common module with printed PCB antenna 2×4 2.54 mm pitch header
 built-in PCB-printed antenna 2 GPIOs: GPOI 0 2 UART RX/TX RESET and CH_PD VCC + GND 14.2 mm W x 14.2 mm L ESP-201 Breadboard friendly! Many I/O available: ISP, PWM, 9 I / O, AD, UART, I²C. The experimenters board has some nice I/O as a buzzer, realy, sensor and has USB serial included. ESP-07 with I/O adapter plate to make it breadboard friendly. All I/O available: GPIO 0/2/4/5/12/13/14/15/16, ADC. NodeMCU, in fact an ESP-12 with USB serial and power regulator for 3.3V included. The NodeMCU placed into the WiFi Smart Car Kit, nice experiments!