The ESP-03 is popular with more advanced users of the ESP8266 as it is the version of the module that has the most available GPIO pins broken out from the chip, which makes it great for more complex projects. Thanks to its on-board ceramic antenna, it's able to be smaller than the ESP-01 module.
7 dedicated GPIOs.
Serial comms (or 2 additional GPIOs).
PWM, I2C, UART, OneWire.
On-board WiFi + ceramic antenna, 802.11 b/g/n, 360m range.
Able to become a wireless access point.
Lua firmware available.
Able to run a web server.
Dimensions: 17.4 x 12.2mm.