The Websocket API server runs on port 4444 and a settings window is available in "Websocket server settings" under OBS' "Tools" menu. The obs-websocket protocol is documented in PROTOCOL.md.

Here's a list of available language APIs for obs-websocket :

Javascript (browser & nodejs) : obs-websocket-js by haganbmj

There's currently no frontend available for obs-websocket.

Possible use cases

Remote control OBS from a phone or tablet on the same local network
Change your stream overlay/graphics based on the current scene (like the AGDQ overlay does)
Automate scene switching with a third-party program (e.g. : auto-pilot, foot pedal, ...)

Build instructions

You'll need QT 5 with QtWebSockets, CMake, and a working development environment for OBS Studio installed on your computer.

Windows

In cmake-gui, you'll have to set the following variables :

QTDIR (path) : location of the Qt environment suited for your compiler and architecture
LIBOBS_INCLUDE_DIR (path) : location of the libobs subfolder in the source code of OBS Studio
LIBOBS_LIB (filepath) : location of the obs.lib file
OBS_FRONTEND_LIB (filepath) : location of the obs-frontend-api.lib file

Linux

On Debian/Ubuntu :

sudo apt-get install libqt5websockets5-dev
git clone --recursive https://github.com/Palakis/obs-websocket.git
cd obs-websocket
mkdir build && cd build
cmake -DLIBOBS_INCLUDE_DIR="<path to the libobs sub-folder in obs-studio's source code>" ..
make
sudo cp obs-websocket.so /usr/lib/obs-plugins
sudo cp -r ../data /usr/share/obs/obs-plugins/obs-websocket

OS X

To do