The OpenWrt Project is a Linux operating system targeting embedded devices. Instead of trying to create a single, static firmware, OpenWrt provides a fully writable filesystem with package management. This frees you from the application selection and configuration provided by the vendor and allows you to customize the device through the use of packages to suit any application. For developers, OpenWrt is the framework to build an application without having to build a complete firmware around it; for users this means the ability for full customization, to use the device in ways never envisioned.
VoCore is open hardware and runs OpenWrt/LEDE. It has WIFI, USB, UART, 20+ GPIOs but is only one inch square. It will help you to make a smart house, study embedded system or even make the tiniest router in the world.