Google Calendar has this feature.
I want to schedule a meeting and find a time where everybody is free. So I select all the people I want to attend and the calendar shows me when everybody's free.

For meetings, where lots of people with lots of other stuff on their schedules come together - which is just about every meeting as far as I am concerned - we are really missing this.

At the moment, I would have to export a bunch of calendars, overlay them and then look for free spaces. So we don't do that and just use a HUGE Doodle or talk 10 minutes as a group about the scheduling instead, both of which is annoying.

Of course it get a bit complicated regarding calendars I am not allowed to view. I think it would be best if there were an on/off switch for taking events not visible to the current user (who is creating the new event) into account.