Meet the Starling: a WiFi-enabled, open source LED display that lets you display custom messages to the world. This device has a modular design and a mobile-friendly app that lets you customize font, scrolling speed, and other settings. The device is compatible with IFTTT and REST APIs.
Starling has an open source hardware. You can stack LED matrices to form larger displays. Starling One, Five, and Ten are currently available for short and longer messages. More information is available here.