From Eltima Software
Serial Port Monitor is a professional application for RS232/422/485 COM ports monitoring. It monitors, displays, logs and analyzes all serial port activity in a system.
It can effectively be used for monitoring serial data exchange between serial devices and any Windows application, debugging computer connection with peripheral devices using COM port - such as modem, mini-ATS, projector, etc, for solving problems with serial (RS232/422/485) data communication control networks and devices, for software and hardware testing and much more. It can monitor any software/hardware that works with a serial port, intercept and record all serial input/output control codes (IOCTLs), tracking their full details and parameters. Serial Port Monitor supports all COM-port types: standard on-board ports, extension board ports, SERIAL-via-USB adapters.
Full features list:
Start/stop monitoring the serial port that is already opened by another application (available in Professional Edition only)
Serial Port Monitor is completely compatible with Windows 7 /Vista (both x32 and x 64)
Serial Port Monitor and all of its internal drivers are digitally signed
Any number of serial ports can be monitored simultaneously
Supports all COM-port