Between my wife, myself and our 2-year-old son, we're always juggling our schedules around to coordinate between daycare, work, and all the activities that a pair of young parents manage. Add to that my parents, who also take care of our son every so often, and factoring in their own lives. After playing around with some different methods, we discovered the best method to coordinate all of our schedules was a free one: iCloud.
So here's the problem: every afternoon, my mother picks up our son from daycare, where he spends a few hours with her before my wife can pick him up and take him home. Once every week or so, my mother has an appointment that conflicts with that routine, so we have to pick him up. The problem was that we didn't always find out about these appointments until the day of, and then it was too late to figure out other options.
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)

White Enamel Four-P...
works well we use it all the time, never thought about adding additional external family members that is a good idea.
@PURRPILL - shhh! iPhone users like to pretend Apple thought up all this stuff first!
Google calendar requires google account, check the news to see what kind of company they are turning into and if you still want to hand them over all your data.
I actually have my google calendar synced with my icloud. My husband has an android - I don't think he's set it up on his phone get but we can also be "in sync" via iCloud via google calendar. If that makes sense...:p
@GRCEVSKI - you are right.. I don't want Google knowing that my daughter has violin lessons on saturday mornings...
Did someone say N'Sync?
I use google calendar and do not have a google account or gmail account. I also use an android phone without a google account. It can be done!
I chose Google calendar over iCal because it is easier to coordinate with non-iPhone users.
All that is needed is a gmail account and the proper set-up for read and right privileges.
I also like, that I could easily access the calendar through the website from anywhere in the world w/o needing to pay for potential roaming charges on my mobile.