Actually the ESP8266 pins that can be used by end users is not so much. There the SPI/I2C/UART, and few pins could be used.  The IO0 can be also used as a normaly IO port, the MCU detect IO0 state to enter the programming/working state. there some projects needs another software UART to communicating with other module such as a GPS module (the default one need to connect the PC for debugging info), as there on-board OLED,  this situation could be easier...

Schematic Released with eagle...