I'm looking for a keyboard shortcut on Mac OSX to show/hide the application toolbar.
I was not able to find something about this issue... Maybe I was searching for the wrong keyword?
Any help is very appreciated.
Depends on the Application. Some do not allow it (like Safari) while others do, but use different shortcuts. For instance, Preview uses ⌘B, whereas Adium uses ⌘⇧T. The best way to find out if an Application supports it is to use the Help menu to search for Toolbar.
If you want to make the shortcut more consistent across applications:
That works even in some apps that don't have a menu bar item for toggling the toolbar (but do have a context menu item for it).