Sunrise Calendar is the best calendar tool you'll use on your desktop and mobile phone. It works with all of the major calendars out there: Google Calendar, iCloud, Exchange, Outlook.com, and Office 365.
It also integrates with everything else you use:
Here's a full list of accounts Sunrise can use.
My favorite part is that you can show google multiple calendars at the same time. If you have a personal calendar and a work version, they both show up in the same view.
Did I mention, it's FREE! There are no ads on the website or the apps.
I use the iPhone app every day.
The iPhone app uses an innovative way to find a time to meet with other people. Check it out:
Some of the other apps I've tried don't have a great website to view and edit my calendar or their phone app isn't powerful enough. Sunrise is the first to get it all to work seemlessly.