SyncML

Many of the mobile phones that you can purchase have support for this little known standard called SyncML that allows for mobile devices to sync with your different PIM (Personal Information Management) applications. Up until now not many people have actually made much use of SyncML but with the J2EE application Sync4J you can easily host your own SyncML server.

Links
SyncML Technical Standards - Open Mobile Alliance Website
Sync4J - SyncML Server