Free-Busy

Data from your calendar to make available to others for Group Calendaring. Doesn't include any of the details of what you're doing, just whether a time slot is already taken.

I used Tungle when it was available, then switched to Doodle. http://www.doodle.com/

• Sept'2014: grr just discovered that free version of Doodle no longer supports integration with your primary calendar app! I don't use it often enough to want to pay.
• May'2018: Free offering includes Calendar integration (again?). Also, their MeetMe page feature is a nice generic way to share a URL with a bunch of people that they can request times with you.
  • hrm the UX is horrible if you're letting people who don't know you schedule times, because it doesn't even pass you their email address, or give you an in-app way to contact them!

2021: Calendly is the new hotness.