The included REST API allows you to make you own client, using HTTP REST calls. You can you use it to achieve several tasks :

Errors returned by the WebAPI are understandable when the HTTP call is not valid :

Get GPIO function

Set GPIO function

Get GPIO value

Set GPIO value

Output a single pulse

Output bit sequence

Output PWM with a duty cycle ratio

Output PWM with an angle for servos

Call a macro on the server

Get full GPIO state/configuration