ESP8266 was initially intended as a serial port to wifi bridge, but can also be used as standalone microcontroller. There is even support for Arduino IDE.
For first projects get a development board like "Wemos D1 mini" (quite a few GPIO pins, requires some easy soldering) or "NodeMCU" (more GPIO pins, comes with presoldered pin headers).
- AT firmware, turning the ESP8266 into a serial/wifi bridge
- NodeMCU firmware, allowing Lua programming
- Arduino port (documentation), using the popular Arduino/C programming language
- MicroPython firmware
- Basic firmware
- ESPlorer integrated development environment mostly useful for AT, Lua and MicroPython development
- Arduino integrated development environment
- ESPtool Python package
If you just need a superfast cheap microcontroller without wifi, check out nodsk8266.