Am Mobile

GCalSync allows you to synchronize your Google Calendar and your mobile phone. Download events to your phone, or add an event on your phone and upload it to Google Calendar.
GCalSync 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)

Supports:

Nokia
3152, 3155, 3155i, 5140i, 6060, 6101, 6102, 6152, 6155, 6155i, 6230i, 6235, 6235i, 6265, 6265i, 6270, 6280, 6611, 6630, 6680, 6681, 6682DP2, 6822, 7260, 7700, 8800, 8801, 9300, 9500, E70, N70, N90, N91, any Series 40 Development Platform 3 and later, any Series 60 Second Edition and later, any Series 80, any Series 90

Motorola
E770, E1070, L6, PEBL U6, RAZR V3 (CLDC 1.1), SLVR L7, V3x, V360

Sony Ericsson
K600, K608, K750, V600, W550, W600, W800, Z520, any JavaPlatform5 or newer

BlackBerry
Any, with software version 4.0 or newer

To install, visit GCalSync with your phone's browser.
OR Download
For blackberry:
Blackberry GCalSync.
Download for other j2me supported devices:
GCalSync.jar Signed file works on all cell phones.