and the new http://xpra.org/. But it's cross-platform, and very easy to use - shocking in today's world of hard to configure and complex software.
The main goal is to start an app on a machine, and than continue to use it on a different machine. I start Eclipse and chrome on my main desktop, than when I use the laptop I just 'attach' to them. Speed is pretty good, certainly faster then running it on the laptop ( which has far less RAM ).
Xpra is similar with VNC - it starts a dedicated Xvfb server for each application to keep them isolated. The wrapper and Xpra are python - I looked briefly, pretty clean code.
On the 'easy to use' - uses mDNS for discovery, it creates the needed SSH tunnels between machines transparently. Each machine runs a small server that handles the attach/detach and additional ports.
They provide easy instructions to install on mac, windows and many versions of linux.
Issues: I'm not sure I fully understand the authentication and the protection of various ports. The firewall should help a bit, but I need to look more into the configs.
I also think using XMPP or some other common protocol for the machine-to-machine communication would have been better.