Open source and completely free of charge
OS-independent standalone application (Java 1.5)
Bidirectional sync between iCalendar and Google Calendar
View and edit your Google Calendar offline
Supports 'Google Apps for Your Domain'
Supports multiple file listeners with multiple Google Accounts
Makes Gmail contacts accessible via LDAP
Customizable Gmail notification
Converts RSS/ATOM feed to iCalendar format
Reusable mail transfer agent
Remote control your computer via Gmail
Multi-user abilities (up to 30-40 HTTP and LDAP clients)
Run in service mode on Windows NT/2000/XP
Run in embedded mode in a Servlet Container
Low memory usage (10-20 MBytes)
Automatic and fast (3-5 sec/sync)
Customizable logging system (Log4J / CommonsLogging)
Ability to do TCP/IP hostname/address filtering

Architecture of the HTTP-based synchronizer

Architecture of the file-based synchronizer

Architecture of the Gmail Address Book / LDAP provider

Architecture of the feed converter

Architecture of the feed cache

Architecture of the Gmail notifier

Architecture of the mail transfer agent

Architecture of the Gmail terminal service