Baoshi wrote a MQTT client for Espressif’s RTOS SDK:
If you search MQTT and ESP8266 on the intraweb, most likely all hits can be traced back to the great work done by TuanPM. However Tuan’s code is based on Espressif’s NON-OS SDK. There has been some great debates about embedded programming with-or-without an OS. To me programming with OS vs NON-OS is like programming with C vs Assembly. I like programming in C, so I wrote a new MQTT client for Espressif’s RTOS SDK.
The source code is available at Github.