The Seeeduino Lite is functionally compatible with the Arduino Leonardo and employs the same ATMega32U4 MCU with a USB interface baked right in. It can be used in places where an Arduino Leonardo is called for the in the Exploring Arduino book. Since no secondary USB-Serial converter IC is required, you can easily use the Seeeduino Lite to emulate arbitrary USB devices, such as a keyboard or mouse. The board has 14 digital input/output pins (of which 7 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB micro B connection, a power jack, an ICSP header, and a reset button. You can learn more about the specifications of the Seeeduino Lite on the Adafruit product page, or at the Seeed studio wiki.
Buy the Seeeduino Lite from Amazon, Adafruit, Seeed Studio, or Mouser.