Good RS232-based Protocols for Embedded to Computer Communication
Some embedded system protocols,
several of them extremely simple,
are listed at
Embedded Systems: Common Protocols,
including:


Tiny Embedded Network (TEN)
Microcontroller Interpreter for Networked Embedded Systems (MINES)
Yet Another Scalable Protocol (YASP)
Local Interconnect Network (LIN)
Serial Servo Controller (SSC)
Robot Operating System serial (rosserial)
netstring
various field busses
Modbus
Controller Area Network (CAN)
Firmata  (Thank you, jippie!)


Perhaps one of these protocols would be adequate for your application as-is, or with only minor tweaking.