Xpra is 'screen for X': it allows you to run X 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.
Xpra is "rootless" or "seamless": programs you run under it show up on your desktop as regular programs, managed by your regular window manager.
Sessions can be accessed over SSH, or password protected over plain TCP sockets.
Xpra is usable over reasonably slow links and does its best to adapt to changing network bandwidth constraints, especially with x264 and automatic settings.
Xpra is open-source (GPLv2+), multi-platform and multi-language, with current clients written in Python and Java.