GCalSync is an open source application that lets you do a two-way synchronization between Google Calendar and your phone’s built-in calendar. It will install on any Java MIDP 2 based mobile device such as Nokia, Motorola, Blackberry or Sony Ericsson.

Features of this application are:

-Download events from Google Calendar to your phone’s calendar
-Upload events created on the phone
-If an event has been changed in Google Calendar since last sync it will be updated on the phone
-Specify time period you want to sync as number of past days (default: 1) and number of future days (default 30)