There are two software packages which communicate with your Visor. The first is coldsync, and the second is pilot-link. These programs are command-line based, and do "low-level" synchronization of the actual databases. They provide [different] interfaces for conduits. I assume you know how to download and install programs yourself. If not, learn. (I suppose you could cheat and install a pre-compiled package, but I don't support that ;-).
coldsync can be found at http://coldsync.org/.
pilot-link can be found at http://www.pilot-link.org/. If you plan on using a Palm Desktop Equivalant, you'll want to use this package.
At this point we should be able to test to make sure things are working. Of course, the visor needs to be in the cradle, and the cradle needs to be plugged into your USB port. The proper modules should be loaded.
NOTE: you MUST press the hotsync button BEFORE running the software. The visor driver will make an entry in syslog, and, eventually (ie, in newer drivers), to /proc/drivers/visor. Assuming the driver connects to /dev/ttyUSB0 and /dev/ttyUSB1, you may run your program to backup the visor (chose the appropriate program):
cd ~ mkdir visorbackup coldsync -p /dev/ttyUSB1 -mb visorbackup pilot-xfer -p /dev/ttyUSB1 -b visorbackup
If you get an error such as "unable to bind to the port", you probably need to re-read the previous paragraph. With any other errors, please refer to the Troubleshooting Guide.
There are a few Palm desktop equivalents. I haven't had time to test them. I intend, at some point, to put something about them in here. If you want to write up something to have it included here, please feel free to send it in. To my knowledge, all of them use the pilot-link libraries to talk with the Visor.
Here is the list that I know of: