Xpra is known as "screen for X" : its seamless mode 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(s), without losing any state. Effectively giving you remote access to individual graphical applications.
Is it open source? - Yes. GNU General Public License v2.0
Development moved to github: https://github.com/Xpra-org/xpra
Latest release 14 days ago: https://github.com/Xpra-org/xpra/archive/refs/tags/v4.2.2.tar.gz