The "LPCXpresso": a tiny circuit board that contains the following:
-ARM Cortex-M3 120mhz processor
-512kb flash
-64kb ram (SRAM)
-ethernet interface
-USB interface, also multiple UART, I2C, SPI, SSP, CAN, PWM...among others.
Also, PWM support for three-phase motor control, reduced power modes (like sleep), integrated JTAG debugger.
Sounds expensive? It's $30. Thirty US dollars. And the whole thing is 35 x 140 mm in size.

Link to product page: http://www.lpctools.com/lpc1768.lpcxpresso.aspx