There are lots of (some would argue way to many) electronic calendars and address books around: they come in Phones, PDAs, as part of office applications, as part of browser applications and as part of numerous web applications like Google calendar or the Horde application suite.
Replication of the data can help to cope with these various information sources. However most replication applications are proprietary and limited to individual devices.
SyncML is a platform independent standard for information synchronization. A wide variety of mobile phones and PDAs support this standard for replication of address books and calendar data.
Currently Horde contains a SyncML module in alpha development status. It allows synchronisation of phones and PDAs with Horde.
Project ideas are: