LKA

With LKA you can define your own packets/messages and send them through the serial port or by TCP/UDP/IP and then see what response you get.

You can also build an cable and monitor two devices communicating through RS232.
There is also three different monitor modes. "Serial Monitor" can monitor/spy on an another application using the serial port, "Serial Internal Monitor" (Win2000/XP only) is like "Serial Monitor" but also has timestamps and "Serial External Monitor" (Win2000/XP only) uses two serial ports and an external cable to monitor/spy on two units that are communicating.
Other features of LKA is checksum calculation and terminal mode.