Google Calendar is probably one of the most successful Web-based calendar tools to hit the Web 2.0 scene. It has a very clean, easy-to-use interface, Gmail integration, the capability to share calendars with others, SMS notification reminders, and--like other Web-based calendars--immediate accessibility regardless of what computer you're using (as long as you have net access). But perhaps the most useful aspect of Google Calendar is that you can carry it with you on your mobile device, as long as you know how.
There are basically two ways to access or sync Google Calendar with your phone. The first is to sync your Google Calendar with the calendar software on your computer, and then sync the calendar information to your phone, using the computer as an intermediary. If you use Outlook to sync up calendar information, Google has conveniently offered a Google Calendar Sync tool for use with Outlook that allows two-way syncing. Aside from Google's syncing tool, there are other third-party Google Calendar syncing applications such as OggSync, Jotlet, and Calgoo, though many of them aren't free. From there, you can sync Outlook with your phone, and you're ready to go. For the Mac, I have had great luck with Spanning Sync, a third-party application that will sync up Google Calendar with iCal. Similarly, you'll then have to sync the calendar information to your phone via the computer.
Then there's the second method of accessing Google Calendar with your phone, and that's over-the-air. The most obvious way to do this is to just head to http://mobile.google.com/calendar using the phone's built-in Web browser. From there, you can add and remove events just as you normally would, but with a scaled down user interface. If you have an iPhone, you'll be treated to a specially formatted mobile Google Calendar page with a user interface optimized for the iPhone's touch-screen display. However, this doesn't always work if you don't have a reliable Internet connection, plus it can be painfully slow. Thankfully, the way around this is to sync Google Calendar with the built-in calendar on your phone.
If you have a BlackBerry, Google has developed an application called Google Sync for BlackBerry, which will sync Google Calendar with the BlackBerry's built-in calendar. If you have a Windows Mobile phone, third-party applications such as OggSync for Windows Mobile/Pocket PC will work. However, if you're one of many thousands of people with just a regular phone, GooSync is probably your best bet when it comes to over-the-air synchronization of your Google Calendar and your phone's built-in calendar. GooSync has a free service as well as a premium service (The premium service is $39.50 a year and offers multicalendar support, a contacts manager, and more), and it supports a variety of phones. You should check out their Supported Devices page to see if your phone is on the list.
As with any of these applications, your mileage may vary as to how well they work, depending on the kind of phone you have, your net connection, and so forth.
If you have any more ideas on how to sync Google Calendar with your mobile device, let us know in the comments.