Starting a graphical instance of a remote application on your *local* machine.Edit
This is kind of weird, so let me try to explain.
Say I have a remote machine that has an application on it that I want to run. And suppose it is a graphical application, something like Sublime Text, as opposed to something CLI like vim.
How could I use this application remotely? Well maybe you've used VNC or remote desktop in the past. Those work, but what if I don't need the whole desktop? This command is super cool. When you tell the remote computer to start your remote application (via ssh), instead of starting its GUI on the remote machine, it starts the program's interface as an X11/Xorg/XQuartz session on your local machine!
ssh -X firstname.lastname@example.org # notice this -X flag!
It's cool because you aren't having to browse around on a remote desktop, manually opening things and waiting for VNC's atrocious network latency. In my experience, this is much faster, and the application behaves natively. You can Command-Tab between it and your other open applications as if it were running on your OS.
I've used this to help set up my and play around with my Raspberry Pi, so I know this works using a Linux server and a Mac client. I'm fairly sure Linux -- Linux works (they both use X), but I'm not sure about a Mac server to Linux client or Mac --> Mac (Apple does not use X11 by default, you have to start it).