How can I use an old mac as an external display using software? I want to use a "space" or "application" with another computer on the network. Just the UI not keyboard and mouse.

The other goal is to add a pseudo screen, without getting an extra monitor. Can't use os x native screen sharing as that will always have the same content as current action space.

Any ideas..?

old mac  = imac 2009, so cannot use as a targetted display. can use hardware to make it happen.

Similar to Can I use my early 2009 iMac as an external screen?

also similar to http://synergy-project.org/
It seems ScreenRecycler does what you need. There's a trial version available, so you can try it.