Is there any Gantt Chart application for Mac OS X that allows dealing with multiple projects?
I have tested Omniplan, which is very good for a single project, but can not work efficiently with multiple projects in a single view.
Could you explain what you mean by "dealing with multiple projects"? Do you mean manage multiple projects in their own files and at the same time have an overview about all of them?
Task Juggler is free and open source:http://www.taskjuggler.org/
it should also be able to manage multiple projects, see https://groups.google.com/forum/#!topic/taskjuggler-users/qYSJoOND3aw
Rational Plan Multi Project sounds as it could be what you need: http://rationalplan-multi-project-for-mac.apponic.com/mac/
Did you try out Merlin http://www.projectwizards.net/en/merlin/
However, I've been working with none of them myself.