Using XForwarding, you can access GUI applications over ssh between two Xorg-powered machines (and sometimes even from a windows machine).
Is there a way to access OSX applications (like Finder) from an Xorg machine?
Finder doesn't use X APIs, so can't be forwarded over over ssh like that. Same with most mac applications; apple have their own windowing system called Aqua. Sharing the desktop via VNC or apple remote desktop works fine though -- look in the "Sharing" preference pane for the "Screen Sharing" option to set it up on the mac, then use a vnc client on the other machine.