The USB armory from Inverse Path is an open source hardware design, implementing a flash drive sized computer.
Hardware is great:
Freescale i.MX53 ARM® Cortex™-A8 800Mhz, 512MB DDR3 RAM
USB host powered (<500 mA) device with compact form factor (65 x 19 x 6 mm)
ARM® TrustZone®, secure boot...