BeagleBone Black (BBB) is an open-hardware low-cost board designed for developers and hobbyists. This board has been developed by the BeagleBoard organization, a is non-profit compromised with the development of open hardware with a huge community of developers and users.
This specific board has become very popular due to its low cost (~35€/~55$) and high features:
It can boot Linux (Debian, Ubuntu, Angström distros) in 10 seconds thanks to the Cortex-A8 processor, is able to work as a stand-alone computer, it has real-time 2x PRU microcontrollers, its large connectivity options (USB, Ethernet, HDMI, SPI, I2C, GPIOs, ...) and large amount of capes available.
If you would read more about the BBB check the BeagleBoard's web.