GMobileSync: Sync Google Calendar with a Windows Mobile device
GMobileSync's not the only fish in the sea here. GooSync is another free program that does basically the same thing. But in our rigorous Download Squad testing, Jason Clarke found it to be somewhat unreliable.
If you're looking for a far more complex solution, you can try to sync Google Calendar with Microsoft Outlook, and synchronize Outlook with your mobile device using ScheduleWorld.
I'm just glad to see folks working on solutions that let you back sync your PDA to a web service of your choice. Mac and Linux users have been left out in the cold by Microsoft when it comes to synchronization with a Windows Mobile device. Programs like The Missing Sync have stepped in, but there's really no reason you should tied down to synchronizing your data with Outlook or other desktop calendars if you'd prefer to use an online system for personal information management.