...A popular design is the 7-key chorder, where one key is used for each of the index finger, middle finger, ring finger and pinky, and 3 keys for the thumb. This can be used with either the left or right hand (or a chorder can be used for each hand, effectively doubling the typing speed).
SpiffChorder hardware is based on Atmel AVR ATmega168, using Objective Development's V-USB (a software-only implementation of a low-speed USB device).