Business-oriented calendar app for OS X I'm looking for a calendar app for OS X that shows available times for meeting attendees, and allows me to propose meetings and accept such proposals from others.

I've been trying Mac calendar and BusyCal, and neither of them meet my needs.

We're using Google app as our calendar server and although I can always use their web interface I prefer to use an installed application.
If you have an Exchange server, Outlook will do that. Calendar for OS X can do it too, provided your calendar server supports it.  OS X Server's Calendar server does.

In either use case the parties must be in the same organization or otherwise accessible due to federation.