Serial / Com Port monitor for Windows x64
UPDATE in 2020: This very old question was relevant in 2010, but is now obsolete. There are plenty of open source serial monitor tools nowadays, notably the Arduino serial monitor tool, just to name one. This question can probably be deleted as it is no longer 'a problem.'
My original answer from 2010: The only one I have found for x64 is "Advanced Serial Port Monitor," which is really quite good. Not free, priced at $60 USD for a single user license. There is a 15-day trial. Honestly, I've been hoping (for quite a while now) that Mark R. would release a 64-bit version of portmon.
Old question, but very relevant. While serial ports may be gone from consumer devices these days, it is still very much used in industrial hardware, point of sale hardware, radios, etc. In my case I was trying to capture a communications session from a configuration software tool for some hardware equipment.
In the end I found a commercial app from Eltima Software called Serial Port Monitor which did the trick for me on Windows 7 x64.