Doodle - Easy meeting scheduling 

Link: http://www.doodle.com/

Doodle eliminates the chaos of scheduling and saves you a lot of time and energy when you’re trying to find a time to bring a number of people together.
Create and participate in meeting polls directly from your Doodle calendar view without the need to switch back and forth between your calendar. You can connect as many calendars and address books with Doodle as you wish, such as your Google, Office 365, Outlook, or iCal calendars. It is easy and free!
Instead of using just one option, Doodle enables you to propose several dates and times and the participants can indicate their availability online.