How to use the laptop as a 2nd monitor

I have tried looking for a solution for this for a long while.

Here's some background information:

I use Linux on all my desktops/laptops. I have a more laptops that I can possibly use, so I hope to use them as an external monitor to increase working screen area.

Some methods I have experimented with:

• Synergy and its variants: This might work well for sharing keyboard and mouse between PCs, but is all, what I wish to do is to use the spare screen area on my laptops for extending windows from my desktop to the laptop. While it is possible I could open another window localised on the laptop, but I can't drag and drop IDE features from one computer to another.

• VNC+Synergy: The VNC option wasn't too helpful as it replicated the desktop, and did not extend it.

• FreeNX: This was the closest I came to sharing desktops, right until I realised FreeNX spawns a whole new session with an entire brand new desktop on the remote PC, and it doesn't extend/expand the current working area.

Essentially, I'm looking for a software solution. Is there an alternative I've missed out on?

Can FreeNX be configured to extend the current desktop into a 2nd monitor configuration?