Google (including Google Apps) Calendar users who use OS X can make use of Spanning Sync to keep their iCal in sync with the Google equivilents. As well as syncing with iCal your Address Book contacts will also make it across.
If you have an iPhone or iPod Touch and want to have read & write access to your calendar then you'll need to make use of this software. Unfortunately due to limitations the CalDav output of Google Calendar results in a read only calendar on the iPhone/iPod Touch.
The service costs $25 per year (with renewals reduced to $15) or $65 for a lifetime license and is licensed per user and not per machine. This means that you can run it on all the machines in your house without issue. Pretty nice of them.
If you'd like to save $5 feel free to use WWVYVS at checkout or click here.