While not open source, there are free(mium) solutions like Tungle (or doodle, etc) do this quite well. Tungle even allows you to do this with people that aren't

While not open source, there are free(mium) solutions like Tungle (or doodle, etc) do this quite well. Tungle even allows you to do this with people that aren't on the same exchange server Disclosure: I work at Tungle