I've made a small script to download the list of the latest available firmwares from Garmin.
Here's the the archive: [Only registered and activated users can see links. ]

The script uses Windows Scripting Host, so it will only run in Windows.
Currently, the script only checks the firmwares for the devices with the support of JNX maps, but this can be changed via GarminUpdateRequest.txt file, which simply lists the necessary part numbers.