This is the most simple USB-Relay possible. It Uses an ATTYINY85 Digispark Board and an simple Chinese relay to complete the Task.
Request: 1x char
TURN_OFF = "0" or 0x30
TURN_ON = "1" or 0x31
TOGGLE = "2" or 0x32
GET_STATE = "3" or 0x33
IDENT = "9" or 0x39
Answer: 1x char
STATE_OFF = "0" or 0x30
STATE_ON = "1" or 0x31
IDENT = "r" or 0x72