Kind of a broad question, don't you think?....
Are you referring to the actual PDA operating system? (Palm OS, Windows CE, etc)
Or are you referring to software used to communicate between a desktop and a PDA?
On my RedHat install I have seen tools used to sync with a Palm device. For a Pocket PC device there is Synce
. And of course there is ActiveSync if you're using a Windows desktop.
With a more specific question, it's difficult for anyone to help you.