OSX2X is a little application to control a remote X server using your local OSX machine as a client. It’s similar to VNC in this regard, but you don’t need to run a local X server to utilize it, and, of course, it only speaks the X11 protocol.

Just for giggles, I tried to use it while attached to my local X11 processes under Apple’s X11 Beta 3. It’s counterintuitive when it’s the local display, but otherwise, it’s pretty cool. :)