Xpra started as screen for X: it allows you to run X11 programs, usually on a remote host, direct their display to your local machine, and then to disconnect from these programs and reconnect from the same or another machine, without losing any state.
It gives you remote access to individual applications. You can see it in action here: screenshots
It now includes much much more than that too, see wiki/Enhancements.
Sessions can be accessed over SSH, TCP, SSL etc
Xpra is usable over reasonably slow links and does its best to adapt to changing network bandwidth constraints and application behaviour.