xpra
Essentially screen or tmux for X:
Xpra gives you “persistent remote applications” for X. That is, unlike normal X applications, applications run with xpra are “persistent”–you can run them remotely, and they don’t die if your connection does. You can detach them, and reattach them later–even from another computer–with no loss of state. And unlike VNC or RDP, xpra is for remote applications, not remote desktops–individual applications show up as individual windows on your screen, managed by your window manager. They’re not trapped in a box.