The Spark Core is a tiny Wi-Fi enabled developing board based on the idea of the Arduino. The Spark Core uses the same Wiring programming language of the Arduino, enabling you to easily port projects over. With it being Wi-Fi, there's no need to connect it to your computer to program it - it all can be done wirelessly (although there is a micro USB socket on board).
The board features an 32-bit STM32 72MHz ARM Cortex M3 processor, 128KB of flash memory, and 20KB of RAM. It also has 2Mb of external flash memory, and UART, SPI an I2C connections. There are 8 digital I/O pins, 4 PWN pins and 8 analog pins which is enough to get some serious Wi-Fi projects up and running.
There are 4 shields already built for it (a shield to convert the footprint to that of an Arduino, a relay shield, a JTAG shield and a battery shield). There are also a few projects up and running, including a wireless motion detector, a solar powered security camera and a Wi-Fi RC car. As with a lot of open-source hardware, this is funded via Kickstarter. If you back them now for $39, you can get a Spark Core in (estimated) September.
UPDATE: The Spark Core is available now.