Image courtesy of Pololu
The Pololu A-Star 32U4 Prime LV 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 A-Star board 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 Pololu A-Star 32U4 Prime LV on the Pololu product page.