OggSync 2.0 is a freeware application for Windows Mobile 5.0 based devices that provides a complete two way synchronization service between your Google Calendar and mobile device without any 3rd party involved.

OggSync features:

-Full 2 way synchronization from your mobile phone to your Google calendar
-Support for Google Hosted Domains
-No 3rd party
-Full support for synchronzing recurring/repeating events along with the changes made to them

The Pro version supports multiple calendar sync and always uptodate background synchronization.