GCalSync is a Java application that can be installed on your mobile phone and allows you to synchronize your phone's calendar with Google Calendar.
Supported phones: BlackBerry, Nokia (6230i, 6680, N70 and others), Motorola (A780, A45, E1000), Sony Ericsson (K750, W800, Z520 and others).
I tried it with my Sony Ericsson K750i, but it couldn't connect to google.com. Does it work on your mobile phone?
Update: There's a new version of GCalSync and the application is now open-source.