I want to point your interest to this little board:
The TI MSP-EXP430F5529LP - for $12.99.
It is a USB-enabled and USB-powered MSP430F5529 16-bit MCU.
– 25-MHz System Clock
– 1.8-V to 3.6-V operation
– 128KB flash, 8KB RAM
– Five timers
– Up to four serial interfaces (SPI, UART, I2C)