It is also possible to output an PWM signal on the GPIO pins.
To set a certain PWM level:
http://<ESP IP address>/control?cmd=PWM,<pin>,<level>
This feature can be used to dim lights, control (DC)-fan speeds, etc.
Be aware that the direct GPIO has currently a "heartbeat" issue where flickers can occur. If that is critical to your application you can use the PCA9685