Although there are lots of great options out there, we decided on using iCloud because we all have iPhones (minus our son, naturally), and we always have them on our person. Plus, it's free, so there's no reason not to.
Here's how we got things in sync:
1. Setup an iCloud account: My family already had our account established since the switch to iOS 5, but if you don't, Apple has a handy how-to page ready to go to get you up to speed.
2. Go to iCloud.com, login, and open up your calendars: On the left hand side you've got a list of all of your calendars, ready to go. Next to each one is a white symbol with some lines in it, kind of like the RSS symbol but withan extra line. Click on it to proceed to the next step.
3. Enter the emails of your iCloud family: You'll need to know your family's email addresses, specifically the ones they use for iCloud (which likely end in @me.com). You'll probably want to keep this a private calendar too, so make sure that box is ticked off as well.
That's it! Once you're done, each family member will then have to accept the calendar, then it will automagically sync across all devices. Now, whenever I have a conflict with my folks', wife's or child's schedule, we can all coordinate together without any problems.
(Images: Kevin Whipps)