Google Calendar v5.5.7 is now capable of telling you when you are free thanks to "Find a time", a new rather useful functionality. When you add a new event it will go through your calendar and highlight the vacant time slots, allowing you to either add a new event or, you know, just chill! We suppose Find a time will be mostly useful to folks with busy schedules and not regular Jack and Joes that have just a few events on their calendars.
The update is currently an Android-exclusive one, and we are not sure if it's going to arrive on iOS.
As a refresher, Google Calendar scored another interesting feature two months ago - Goals. Basically, it helps you fit new recurring events in your ever-busy schedule, thus aiding you in achieving your goals.