Google Calendar Sync; a program for synchronizing Google Calendar with Microsoft Outlook.

This update now brings Google Calendar to a new level where user adoption will increase because it now can coordinate with Outlook; the most popular email client in use today. Here is Google's complete tutorial for setting up Google Calendar Sync.
There are Other Options Available
Google Calendar Sync for Outlook rounds out a fair list of apps that allow synchronization with Google Calendar between Outlook, Mozilla Thunderbird, and even Blackberry's.

For Mozilla Thunderbird
• Provider Add-on is a plug-in for Thunderbird that enables synchronization. Here is a great instructional on how to use it to sync the two systems.
Other Related Thunderbird Apps
• BirdieSync is a commercial application that reportedly does a great job of syncing Google Calendar with Pocket Outlook so that you can then have (by extension) your Google Calendar on your smart phone.
• FinchSync does the same thing as BirdieSync except it is free but that reportedly comes with less user-friendliness and reliability.
Synchronize Between Google Calendar and Your Windows Mobile PDA/Smartphone
It is still in beta but GMobileSync offers the ability to synchronize directly between Google Calendar and your portable if it is running Windows Mobile 5.0.